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/api/routes/api/energy.py

@@ -169,7 +169,7 @@ def get_prices(
 
         Response ``points`` carries per-slot:
         - ``buy  = total``              (Tibber all-inclusive price)
-        - ``sell = total − energy_tax − sell_adjust``  (from active version values)
+        - ``sell = total − energy_tax − sell_fee − sell_adjust``  (from active version values)
         - ``level``                     (Tibber price level, may be null)
 
         ``tariff`` is null.
@@ -222,7 +222,8 @@ def get_prices(
         )
         rows = list(reversed(db.execute(stmt).scalars().all()))
 
-        # Derive sell price per-point using version values (energy_tax + sell_adjust).
+        # Derive sell price per-point using version values
+        # (energy_tax + sell_fee + sell_adjust).
         from decimal import Decimal
 
         def _d(v: Any) -> Decimal:
@@ -230,12 +231,13 @@ def get_prices(
 
         energy = version.values.get("energy", {}) if version.values else {}
         energy_tax = _d(energy.get("energy_tax", 0))
+        sell_fee = _d(energy.get("sell_fee", 0))
         sell_adjust = _d(energy.get("sell_adjust", 0))
 
         points = []
         for row in rows:
             total = _d(row.total)
-            sell = float(total - energy_tax - sell_adjust)
+            sell = float(total - energy_tax - sell_fee - sell_adjust)
             points.append(
                 PricePointSchema(
                     starts_at=_as_utc(row.starts_at),