feat: weekly digest + local-time quiet hours

Weekly Digest (send_weekly_digest Celery task):
- Runs every Monday 8:30 AM UTC via beat schedule
- Queries all followed bills updated in the past 7 days per user
- Sends low-priority ntfy push (Priority: low, Tags: newspaper,calendar)
- Creates a NotificationEvent (weekly_digest type) for RSS feed visibility
- Admin can trigger immediately via POST /api/admin/trigger-weekly-digest
- Manual Controls panel now includes "Send Weekly Digest" button

Local-time quiet hours:
- Browser auto-detects IANA timezone via Intl.DateTimeFormat().resolvedOptions().timeZone
- Timezone saved to notification_prefs alongside quiet_hours_start/end on Save
- Dispatcher converts UTC → user's local time (zoneinfo stdlib) before hour comparison
- Falls back to UTC if timezone absent or unrecognised
- Quiet hours UI: 12-hour AM/PM selectors, shows detected timezone as hint
- Clearing quiet hours also clears stored timezone

Co-Authored-By: Jack Levy

backend/app/schemas/schemas.py

@@ -18,9 +18,10 @@ class NotificationSettingsResponse(BaseModel):
     # Digest
     digest_enabled: bool = False
     digest_frequency: str = "daily"  # daily | weekly
-    # Quiet hours (UTC hour integers 0-23, None = disabled)
+    # Quiet hours — stored as local-time hour integers (0-23); timezone is IANA name
     quiet_hours_start: Optional[int] = None
     quiet_hours_end: Optional[int] = None
+    timezone: Optional[str] = None  # IANA name, e.g. "America/New_York"
 
     model_config = {"from_attributes": True}
 
@@ -37,6 +38,7 @@ class NotificationSettingsUpdate(BaseModel):
     digest_frequency: Optional[str] = None
     quiet_hours_start: Optional[int] = None
     quiet_hours_end: Optional[int] = None
+    timezone: Optional[str] = None  # IANA name sent by the browser on save
 
 
 class NotificationEventSchema(BaseModel):