M8-R11: make business timezone deterministic

app/services/timezone.py

@@ -19,8 +19,8 @@ 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.
+2. The DST-aware ``Europe/Amsterdam`` business timezone.  This keeps local-day
+   calculations deterministic when a deployment does not set ``TZ``.
 """
 
 from __future__ import annotations
@@ -40,7 +40,7 @@ def local_tz() -> "tzinfo":
     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``.
+    2. The DST-aware ``Europe/Amsterdam`` business timezone.
 
     **Monkeypatch this function in tests** to get deterministic timezone
     behaviour regardless of CI host configuration::
@@ -51,9 +51,7 @@ def local_tz() -> "tzinfo":
     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]
+    return ZoneInfo("Europe/Amsterdam")
 
 
 def to_local(dt: datetime) -> datetime: