Add timezone-aware daily digest notifications

The daily digest notification count was inconsistent with the kanban UI
because the server used UTC time while the client used local time.
A task due Dec 24 would appear overdue on the server (UTC Dec 25) but
still show as "due today" for the user (local Dec 24).

Changes:
- Add timezone column to notification_preference table
- Auto-capture user's timezone from X-Timezone header when fetching tasks
- Use stored timezone in HandleDailyDigest for accurate overdue calculation

The mobile app already sends X-Timezone on every request, so no client
changes are needed. The timezone is captured on each app launch when
the tasks API is called.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

migrations/011_add_timezone_to_notification_preferences.up.sql

@@ -0,0 +1,7 @@
+-- Add timezone column to notification preferences
+-- Stores IANA timezone name (e.g., "America/Los_Angeles") for background job calculations
+ALTER TABLE notifications_notificationpreference
+ADD COLUMN timezone VARCHAR(50);
+
+-- Add comment for documentation
+COMMENT ON COLUMN notifications_notificationpreference.timezone IS 'IANA timezone name for daily digest calculations (e.g., America/Los_Angeles)';