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>

custom_components/nestquest/core/sweep.py

@@ -39,9 +39,10 @@ lets exactly one of them win the claim — the loser returns [] and
 announces nothing.
 
 The caller supplies ``today`` as a plain :class:`datetime.date`: the
-household-local calendar date, read from ``hass.config.time_zone`` by
-the integration or from the configured household timezone by the API.
-This module never reads a timezone database or config of its own.
+household-local calendar date, which the API reads in the
+admin-stored application timezone (the API host's local zone when none
+is stored).  This module never reads a timezone database or config of
+its own.
 """
 from __future__ import annotations
 
@@ -97,9 +98,9 @@ async def run_missed_sweep(
     """Build one missed event per unswept past-due open instance.
 
     ``today`` is the household-local calendar date the sweep runs for
-    (the caller owns the clock: the integration reads it from
-    ``hass.config.time_zone``, the API from the configured household
-    timezone, falling back to its host's local time when unset).
+    (the caller owns the clock: the API reads it in the admin-stored
+    application timezone, falling back to its host's local time when
+    unset).
     Returns the ``(event_type, payload)`` tuples to announce, EMPTY
     when the watermark already covers ``today`` (a same-night rerun) —
     the caller decides how to publish them and announces nothing on