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/timezone.py

@@ -0,0 +1,100 @@
+"""Server local-timezone helpers.
+
+All time is stored as UTC (no change).  This module provides a single,
+monkeypatch-friendly entry point for converting UTC moments to the server's
+local timezone — used for business-day boundary calculations (standing charges,
+effective-from date interpretation, etc.).
+
+Design goals
+------------
+- Zero third-party dependencies (no tzlocal, no pytz).
+- Testable without depending on CI host timezone:
+  monkeypatch ``local_tz`` to a fixed ``ZoneInfo`` in unit tests.
+- DST-correct: conversions use ``astimezone(local_tz())`` per instant, so each
+  moment is independently correct even during DST transitions.
+- Consistent with the existing ``.astimezone()`` pattern already used in
+  ``homeassistant_inbound.py`` and ``poo.py``.
+
+Priority for resolving the local timezone
+-----------------------------------------
+1. ``TZ`` environment variable — ``ZoneInfo(os.environ["TZ"])``.
+   Set ``TZ=Europe/Amsterdam`` in the deployment env for correct NL handling.
+2. System local timezone fallback: ``datetime.now().astimezone().tzinfo``.
+   This matches the behaviour callers already relied on implicitly.
+"""
+
+from __future__ import annotations
+
+import os
+from datetime import date, datetime, timezone
+from typing import TYPE_CHECKING
+from zoneinfo import ZoneInfo
+
+if TYPE_CHECKING:
+    from datetime import tzinfo
+
+
+def local_tz() -> "tzinfo":
+    """Return the server's local timezone.
+
+    Resolution order:
+    1. ``TZ`` environment variable (``ZoneInfo(TZ)``).  Set
+       ``TZ=Europe/Amsterdam`` in production for correct NL/DST handling.
+    2. System local timezone via ``datetime.now().astimezone().tzinfo``.
+
+    **Monkeypatch this function in tests** to get deterministic timezone
+    behaviour regardless of CI host configuration::
+
+        monkeypatch.setattr("app.services.timezone.local_tz",
+                            lambda: ZoneInfo("Europe/Amsterdam"))
+    """
+    tz_env = os.environ.get("TZ", "").strip()
+    if tz_env:
+        return ZoneInfo(tz_env)
+    # System fallback — identical to the .astimezone() pattern already used
+    # in homeassistant_inbound.py and poo.py.
+    return datetime.now().astimezone().tzinfo  # type: ignore[return-value]
+
+
+def to_local(dt: datetime) -> datetime:
+    """Convert *dt* to the server local timezone.
+
+    Handles three input cases:
+    - timezone-aware (any zone): converted via ``astimezone``.
+    - timezone-naive: assumed UTC (SQLite read-back convention), then converted.
+
+    Returns a timezone-aware datetime in ``local_tz()``.
+    """
+    if dt.tzinfo is None:
+        dt = dt.replace(tzinfo=timezone.utc)
+    return dt.astimezone(local_tz())
+
+
+def local_now() -> datetime:
+    """Return the current moment as a timezone-aware local datetime."""
+    return datetime.now(local_tz())
+
+
+def local_date(dt: datetime) -> date:
+    """Return the local calendar date for *dt*.
+
+    Equivalent to ``to_local(dt).date()``.  A distinct helper to make
+    call sites readable.
+    """
+    return to_local(dt).date()
+
+
+def local_midnight_utc(d: date) -> datetime:
+    """Return the UTC instant that corresponds to local midnight on *d*.
+
+    Example (Europe/Amsterdam, CEST=UTC+2):
+        local_midnight_utc(date(2026, 6, 25))
+        → datetime(2026, 6, 24, 22, 0, 0, tzinfo=UTC)
+
+    This is used to convert a local calendar date back to a UTC boundary, e.g.
+    to compute the start/end of a local day for DB queries.
+    """
+    tz = local_tz()
+    # Build a naive local datetime at midnight, then localize and convert to UTC.
+    local_midnight = datetime(d.year, d.month, d.day, 0, 0, 0, tzinfo=tz)
+    return local_midnight.astimezone(timezone.utc)