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)