Document the manual, admin-set application timezone (task e9aa78f3)

Add an "Application timezone" section to docs/ARCHITECTURE.md: the
timezone is set only by an admin from the admin app (GET/PATCH
/api/v1/admin/settings), is the single source of local time for the
panel, completions, the missed-sweep scheduler and the admin app, means
the API host's local zone while empty (published as the effective zone),
and an admin save of timezone or day_rollover_time re-plans the sweep.

Correct docstrings and comments that still described the local date as
Home Assistant's ("HA-local", read from hass.config.time_zone by the
integration) or the stored zone as reported from Home Assistant.
Comment and docstring changes only; no behaviour change.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_core_snapshot.py

@@ -136,7 +136,7 @@ async def test_build_snapshot_explicit_values(hass, make_entry, tmp_path) -> Non
 
     # Single clock read for the seed/snapshot anchor (no midnight skew);
     # the coordinator no longer owns a clock read (the API serves the
-    # snapshot), so the test resolves the HA-local zone directly.
+    # snapshot), so the test resolves its own local zone directly.
     time_zone = ZoneInfo(hass.config.time_zone)
     today = datetime.datetime.now(time_zone).date()
     # Pin the build instant at 12:00 local: past Ada's 10:00 due time