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/services/contracts.py

@@ -286,6 +286,32 @@ def deactivate_contract(session: Session, contract: EnergyContract) -> None:
     logger.info("Deactivated contract %r (id=%d)", contract.name, contract.id)
 
 
+def active_contract_versions(session: Session) -> list[EnergyContractVersion]:
+    """Return all versions of the currently active contract, ordered by effective_from ascending.
+
+    Returns an empty list when there is no active contract.  The list spans the
+    full history of the active contract (all closed + the current open version)
+    and is used to iterate over pricing-rate segments for cross-version fixed-
+    cost / credit accumulation (Principle C).
+    """
+    active = session.execute(
+        select(EnergyContract).where(EnergyContract.active.is_(True)).limit(1)
+    ).scalar_one_or_none()
+
+    if active is None:
+        return []
+
+    return list(
+        session.execute(
+            select(EnergyContractVersion)
+            .where(EnergyContractVersion.contract_id == active.id)
+            .order_by(EnergyContractVersion.effective_from)
+        )
+        .scalars()
+        .all()
+    )
+
+
 def active_contract_version_at(
     session: Session, ts: datetime
 ) -> EnergyContractVersion | None: