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_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},