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.

app/integrations/pricing/profiles.py

@@ -123,6 +123,7 @@ class ManualProfile(BaseModel):
 class TibberEnergySpec(BaseModel):
     source: str  # must be "tibber_api"
     energy_tax: FieldSpec  # subtracted from total to derive sell price
+    sell_fee: FieldSpec  # verkoopvergoeding (feed-in fee); always subtracted from sell; default 0.0248
     sell_adjust: FieldSpec  # additional sell-price adjustment; default 0
 
 
@@ -268,10 +269,13 @@ def _fill_defaults_manual(values: dict[str, Any], profile: ManualProfile) -> dic
 
 
 def _fill_defaults_tibber(values: dict[str, Any], profile: TibberProfile) -> dict[str, Any]:
-    """Return a copy of *values* with sell_adjust and management_fee defaults applied."""
+    """Return a copy of *values* with sell_fee, sell_adjust and management_fee defaults applied."""
     filled = dict(values)
 
     energy = dict(filled.get("energy", {}))
+    # Apply default for sell_fee (default=0.0248) if absent.
+    if "sell_fee" not in energy and profile.energy.sell_fee.default is not None:
+        energy["sell_fee"] = profile.energy.sell_fee.default
     # Apply default for sell_adjust (default=0) if absent.
     if "sell_adjust" not in energy and profile.energy.sell_adjust.default is not None:
         energy["sell_adjust"] = profile.energy.sell_adjust.default
@@ -347,6 +351,7 @@ def _validate_tibber_values(values: dict[str, Any], profile: TibberProfile) -> d
 
     # Required energy fields.
     _require_numeric("energy", "energy_tax", energy)
+    _require_numeric("energy", "sell_fee", energy)
     _require_numeric("energy", "sell_adjust", energy)
     # Required standing fields.
     _require_numeric("standing", "management_fee", standing)
@@ -361,9 +366,10 @@ def validate_values(kind: str, values: dict[str, Any]) -> dict[str, Any]:
     """Validate a contract-values dict against the named profile structure.
 
     Fields that carry a ``default`` in the profile (e.g. ``ode``,
-    ``sell_adjust``, tibber ``management_fee``) are silently filled in when
-    absent from *values*.  Fields with no default that are absent, or fields
-    whose value is not a number, cause a ``ProfileValidationError``.
+    ``sell_fee``, ``sell_adjust``, tibber ``management_fee``) are silently
+    filled in when absent from *values*.  Fields with no default that are
+    absent, or fields whose value is not a number, cause a
+    ``ProfileValidationError``.
 
     Parameters
     ----------