fix(tibber): deduct verkoopvergoeding (sell_fee) from feed-in sell price

Tibber's API `total` already includes the buy-side inkoopvergoeding
(verified from production data: total = spot×1.21 + energy_tax 0.11085 +
inkoopvergoeding 0.0248). Under net metering Tibber pays back
`total − verkoopvergoeding` per returned kWh (NL: EUR 0.28 -> 0.2552), so the
two EUR 0.0248 fees do NOT cancel — the feed-in price sits 0.0248 below buy.

Model the verkoopvergoeding as a first-class, always-subtracted contract
field `energy.sell_fee` (default 0.0248) instead of folding it into
`sell_adjust`. New sell formula:

    sell = total − energy_tax − sell_fee − sell_adjust

`sell_adjust` now carries only the net-metering energy-tax refund
(= −energy_tax). Applied in both the billing strategy and the /prices
endpoint; recorded in the pricing snapshot. Frontend renders the field
automatically (dynamic profile form). Docs (references, m6) corrected to
drop the wrong "fees cancel" premise.

tests/test_api_energy.py

@@ -363,13 +363,65 @@ def test_prices_tibber_contract_returns_points(energy_client):
     starts_at_list = [p["starts_at"] for p in body["points"]]
     assert starts_at_list == sorted(starts_at_list)
 
-    # Check buy/sell calculations: buy=total=0.245, sell=total-energy_tax-sell_adjust=0.245-0.1108-0.0
+    # Check buy/sell calculations: buy=total=0.245, sell=total-energy_tax-sell_fee-sell_adjust
+    # (this version has no sell_fee/sell_adjust → both default to 0 at read time).
     for p in body["points"]:
         assert abs(p["buy"] - 0.245) < 1e-6
         assert abs(p["sell"] - (0.245 - 0.1108)) < 1e-4
         assert p["level"] == "NORMAL"
 
 
+def test_prices_tibber_sell_reflects_sell_fee(energy_client):
+    """/prices sell price deducts sell_fee (verkoopvergoeding), net-metering config."""
+    client, engine, _app = energy_client
+    _login(client)
+
+    # Net-metering version: sell_adjust = −energy_tax (refund tax), sell_fee = 0.0248.
+    now = datetime.now(UTC)
+    with Session(engine) as session:
+        contract = EnergyContract(
+            name="Tibber NetMeter",
+            kind="tibber",
+            active=True,
+            currency="EUR",
+            created_at=now,
+            updated_at=now,
+        )
+        session.add(contract)
+        session.flush()
+        session.add(
+            EnergyContractVersion(
+                contract_id=contract.id,
+                effective_from=now - timedelta(days=30),
+                effective_to=None,
+                values={
+                    "energy": {
+                        "energy_tax": 0.1108,
+                        "sell_fee": 0.0248,
+                        "sell_adjust": -0.1108,
+                    },
+                    "standing": {"management_fee": 5.99, "network_fee": 25.0},
+                    "credits": {"heffingskorting": 600.0},
+                },
+                created_at=now,
+            )
+        )
+        session.commit()
+    _make_tibber_prices(engine, count=3)
+
+    start = (datetime.now(UTC) - timedelta(hours=2)).isoformat()
+    end = (datetime.now(UTC) + timedelta(hours=2)).isoformat()
+    resp = client.get("/api/energy/prices", params={"start": start, "end": end})
+    assert resp.status_code == 200
+    body = resp.json()
+    assert body["kind"] == "tibber"
+    assert len(body["points"]) == 3
+    # sell = 0.245 − 0.1108 − 0.0248 − (−0.1108) = 0.245 − 0.0248 = 0.2202
+    for p in body["points"]:
+        assert abs(p["buy"] - 0.245) < 1e-6
+        assert abs(p["sell"] - 0.2202) < 1e-4
+
+
 def test_prices_tibber_limit_caps_results(energy_client):
     client, engine, _app = energy_client
     _login(client)

tests/test_pricing_profiles.py

@@ -137,6 +137,11 @@ class TestLoadProfileTibber:
         profile = load_profile("tibber")
         assert profile.energy.sell_adjust.default == 0
 
+    def test_sell_fee_has_default_verkoopvergoeding(self) -> None:
+        profile = load_profile("tibber")
+        assert profile.energy.sell_fee.unit == "EUR/kWh"
+        assert profile.energy.sell_fee.default == 0.0248
+
     def test_management_fee_has_default(self) -> None:
         profile = load_profile("tibber")
         assert profile.standing.management_fee.default is not None
@@ -350,6 +355,19 @@ class TestValidateValuesTibber:
         filled = validate_values("tibber", values)
         assert filled["energy"]["sell_adjust"] == 0
 
+    def test_sell_fee_default_applied_when_absent(self) -> None:
+        values = {
+            "energy": {
+                "energy_tax": 0.1108,
+                "sell_adjust": 0.0,
+                # sell_fee absent — has default 0.0248 (verkoopvergoeding)
+            },
+            "standing": {"management_fee": 5.99, "network_fee": 9.87},
+            "credits": {"heffingskorting": 600.0},
+        }
+        filled = validate_values("tibber", values)
+        assert filled["energy"]["sell_fee"] == 0.0248
+
     def test_management_fee_default_applied_when_absent(self) -> None:
         values = {
             "energy": {"energy_tax": 0.1108, "sell_adjust": 0.0},

tests/test_pricing_strategies.py

@@ -6,7 +6,7 @@ Acceptance criteria covered
 2. Manual strategy: dual-tariff import/export/net calculated correctly (hand-verified).
 3. Manual strategy: Decimal precision — no float binary rounding errors.
 4. Tibber strategy: queries the most recent TibberPrice with starts_at ≤ t0.
-5. Tibber strategy: buy=total, sell=total−energy_tax−sell_adjust.
+5. Tibber strategy: buy=total, sell=total−energy_tax−sell_fee−sell_adjust.
 6. Tibber strategy: negative total → negative export_revenue (not clamped).
 7. Tibber strategy: raises TibberPriceNotFoundError when no matching row exists.
 8. ``register_strategy`` / ``get_strategy`` round-trip works.
@@ -371,6 +371,68 @@ class TestTibberStrategy:
         # sell = 0.25 - 0.10 - 0.02 = 0.13; export_revenue = 2 × 0.13 = 0.26
         assert result["export_revenue"] == Decimal("2") * Decimal("0.13")
 
+    def test_sell_deducts_sell_fee(self, tibber_db) -> None:
+        """verkoopvergoeding (sell_fee) is subtracted from the feed-in price.
+
+        Under net metering the energy tax is refunded (sell_adjust = −energy_tax),
+        so sell should equal total − sell_fee. Verifies the fee is a first-class,
+        always-deducted term and does NOT cancel against the buy-side inkoopvergoeding
+        that is already baked into total.
+        """
+        t0 = _ts(10, 0)
+        with Session(tibber_db) as session:
+            _insert_tibber_price(session, starts_at=_ts(9, 45), total=0.3073)
+            session.commit()
+
+        # Net-metering config: sell_adjust = −energy_tax refunds the tax;
+        # sell_fee = 0.0248 (Tibber verkoopvergoeding) is still deducted.
+        values = {
+            "energy": {
+                "energy_tax": 0.11085,
+                "sell_fee": 0.0248,
+                "sell_adjust": -0.11085,
+            },
+            "standing": {"management_fee": 5.99, "network_fee": 9.87},
+            "credits": {"heffingskorting": 600.0},
+        }
+        with Session(tibber_db) as session:
+            deltas = PeriodDeltas(
+                d1=Decimal("0"), d2=Decimal("0"),
+                r1=Decimal("0"), r2=Decimal("1"),
+            )
+            result = self._call(deltas, t0, session, values=values)
+
+        # sell = 0.3073 − 0.11085 − 0.0248 − (−0.11085) = 0.3073 − 0.0248 = 0.2825
+        expected_sell = (
+            Decimal("0.3073") - Decimal("0.11085") - Decimal("0.0248") - Decimal("-0.11085")
+        )
+        assert expected_sell == Decimal("0.2825")
+        assert result["export_revenue"] == Decimal("1") * expected_sell
+        assert result["pricing"]["sell_fee"] == "0.0248"
+        assert Decimal(result["pricing"]["sell"]) == Decimal("0.2825")
+
+    def test_sell_fee_absent_defaults_to_zero(self, tibber_db) -> None:
+        """A version without sell_fee (pre-migration) reads it as 0 — no silent deduction."""
+        t0 = _ts(10, 0)
+        with Session(tibber_db) as session:
+            _insert_tibber_price(session, starts_at=_ts(9, 45), total=0.25)
+            session.commit()
+
+        values = {
+            "energy": {"energy_tax": 0.10, "sell_adjust": 0.0},  # no sell_fee key
+            "standing": {"management_fee": 5.99, "network_fee": 9.87},
+            "credits": {"heffingskorting": 600.0},
+        }
+        with Session(tibber_db) as session:
+            deltas = PeriodDeltas(
+                d1=Decimal("0"), d2=Decimal("0"),
+                r1=Decimal("0"), r2=Decimal("1"),
+            )
+            result = self._call(deltas, t0, session, values=values)
+        # sell = 0.25 − 0.10 − 0 − 0 = 0.15
+        assert result["export_revenue"] == Decimal("0.15")
+        assert result["pricing"]["sell_fee"] == "0"
+
     def test_uses_most_recent_price_before_t0(self, tibber_db) -> None:
         """Correct row: starts_at ≤ t0, most recent wins."""
         t0 = _ts(10, 0)