FU10: localize energy time math to server tz; accrue fixed/credit per elapsed local day

- Store UTC, compute day boundaries in server local timezone (new app/services/timezone.py).
- summarize: fixed fee / tax credit accrue only for elapsed whole local days,
  integrated across contract versions — no future-day inflation, no version-switch collapse.
- MQTT import_cost_total / export_revenue_total getters delegate to summarize (no inline
  apportionment), anchored at the active contract's earliest version effective_from so the
  total sensors stay monotonic and never jump backward.
- effective_from: naive datetimes interpreted as server-local wall-clock, then stored UTC.
- frontend ContractForm sends local-midnight naive datetime (was UTC midnight).
- get_costs_summary default window uses server-local "today".

app/api/routes/api/energy.py

@@ -75,6 +75,7 @@ from app.schemas.energy import (
 from app.services.auth import AuthenticatedSession
 from app.services.contracts import active_contract_version_at
 from app.services.energy_cost import recompute_range, summarize
+from app.services.timezone import local_midnight_utc, local_now
 
 logger = logging.getLogger(__name__)
 
@@ -358,12 +359,15 @@ def get_costs_summary(
     Both ``fixed_costs`` and ``credits`` are derived from the **currently active
     contract version at ``end``**.  When no active contract exists they are 0.
     """
-    now = datetime.now(UTC)
-
-    if start is None:
-        start = now.replace(hour=0, minute=0, second=0, microsecond=0)
-    if end is None:
-        end = start + timedelta(days=1)
+    if start is None or end is None:
+        # Default to the server's local today: [local_midnight, next_local_midnight).
+        # This ensures "today" aligns with the local calendar day (NL time) rather
+        # than UTC midnight.
+        local_today = local_now().date()
+        if start is None:
+            start = local_midnight_utc(local_today)
+        if end is None:
+            end = local_midnight_utc(local_today + timedelta(days=1))
 
     result = summarize(db, _as_utc(start), _as_utc(end))
     return SummaryResponse(**result)

app/api/routes/api/energy_contracts.py

@@ -44,6 +44,7 @@ from app.schemas.energy_contract import (
     VersionCreate,
 )
 from app.services.auth import AuthenticatedSession
+from app.services import timezone as _tz_mod
 from app.services.contracts import (
     ContractVersionError,
     activate_contract,
@@ -113,6 +114,30 @@ def _raise_422_for_profile_error(exc: Exception) -> Any:
     )
 
 
+def _localize_effective_from(dt: datetime | None) -> datetime:
+    """Resolve *dt* to an aware UTC datetime for storage.
+
+    Rules (Principle A):
+    - If *dt* is None → use ``datetime.now(UTC)`` (unchanged from before).
+    - If *dt* is timezone-aware → convert to UTC as-is.
+    - If *dt* is timezone-naive → interpret as server local wall-clock time,
+      localize with ``local_tz()``, then convert to UTC.
+
+    This means a front-end that sends ``"2026-06-25T00:00:00"`` (no Z) has it
+    interpreted as local midnight (e.g. CEST = UTC+2 → stored as 2026-06-24T22:00:00Z),
+    not as UTC midnight.
+    """
+    if dt is None:
+        return datetime.now(UTC)
+    if dt.tzinfo is not None:
+        # Already aware: convert to UTC.
+        return dt.astimezone(UTC)
+    # Naive: assume server local wall-clock.
+    tz = _tz_mod.local_tz()
+    local_dt = dt.replace(tzinfo=tz)
+    return local_dt.astimezone(UTC)
+
+
 # ---------------------------------------------------------------------------
 # GET /api/energy/profiles
 # ---------------------------------------------------------------------------
@@ -175,7 +200,7 @@ def create_energy_contract(
     The new contract is created with ``active=False``; use
     PATCH /api/energy/contracts/{id} with ``active=true`` to activate it.
     """
-    effective_from = body.effective_from or datetime.now(UTC)
+    effective_from = _localize_effective_from(body.effective_from)
 
     try:
         contract = create_contract(
@@ -284,12 +309,13 @@ def add_contract_version(
     Historical versions are never modified; this endpoint is append-only.
     """
     contract = _get_contract_or_404(db, contract_id)
+    effective_from = _localize_effective_from(body.effective_from)
 
     try:
         add_version(
             db,
             contract,
-            effective_from=body.effective_from,
+            effective_from=effective_from,
             values=body.values,
         )
     except ContractVersionError as exc: