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/strategies.py

@@ -209,12 +209,23 @@ def _tibber_strategy(
         query on ``starts_at``.
 
     Formula (§3.4):
-    - ``buy  = total`` (Tibber's all-inclusive price, already includes tax)
-    - ``sell = total − energy_tax − sell_adjust``
+    - ``buy  = total`` (Tibber's all-inclusive price; already includes energy
+      tax, VAT and the buy-side ``inkoopvergoeding``)
+    - ``sell = total − energy_tax − sell_fee − sell_adjust``
     - ``import_cost   = (Δd1 + Δd2) × buy``
     - ``export_revenue = (Δr1 + Δr2) × sell``
     - ``net_cost = import_cost − export_revenue``
 
+    ``sell_fee`` models Tibber's per-kWh **verkoopvergoeding** (feed-in fee,
+    €0.0248/kWh incl. VAT since 2026-01-01).  It is always deducted from the
+    feed-in payout: even under the net-metering (saldering) scheme, Tibber pays
+    ``total − verkoopvergoeding`` per returned kWh (Tibber NL: "€0,28 − €0,0248
+    = €0,2552").  ``total`` already contains the equal buy-side
+    ``inkoopvergoeding``, so the two fees do **not** cancel — the feed-in price
+    sits ``sell_fee`` below the buy price.  ``sell_adjust`` is a separate manual
+    correction: under net metering it carries back the refunded energy tax
+    (``sell_adjust = −energy_tax``), leaving ``sell = total − sell_fee``.
+
     Tibber does not differentiate tariff slots (dal vs normal) — the 15-minute
     API price applies to the full delivered/returned volume.
 
@@ -248,11 +259,12 @@ def _tibber_strategy(
 
     energy = values.get("energy", {})
     energy_tax = _to_decimal(energy.get("energy_tax", 0))
+    sell_fee = _to_decimal(energy.get("sell_fee", 0))
     sell_adjust = _to_decimal(energy.get("sell_adjust", 0))
 
     total = _to_decimal(price_row.total)
     buy = total
-    sell = total - energy_tax - sell_adjust
+    sell = total - energy_tax - sell_fee - sell_adjust
 
     total_delivered = deltas.d1 + deltas.d2
     total_returned = deltas.r1 + deltas.r2
@@ -269,6 +281,7 @@ def _tibber_strategy(
         "buy": str(buy),
         "sell": str(sell),
         "energy_tax": str(energy_tax),
+        "sell_fee": str(sell_fee),
         "sell_adjust": str(sell_adjust),
     }