feat: per-user timezone-aware briefing scheduler

Briefing slots (4am/8am/12pm/4pm) now fire in each user's local timezone
rather than UTC, so the schedule matches the user's actual day.

Backend:
- briefing_scheduler: replaced 4 global UTC cron jobs with per-user
  CronTrigger jobs keyed to the user's IANA timezone; update_user_schedule()
  live-patches the scheduler when config is saved (no restart needed)
- Catchup logic evaluates missed slots in the user's local timezone
- put_config route calls update_user_schedule() after saving

Frontend:
- New Timezone section in Briefing settings: text input pre-filled from
  browser (Intl.DateTimeFormat) with a Detect button for re-detection
- Slot times shown as fixed local times (4:00 am etc.) with the configured
  timezone displayed beneath, replacing the old UTC-conversion display
- timezone field added to BriefingConfig type and default

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/api/client.ts

@@ -329,6 +329,7 @@ export interface BriefingConfig {
   slots: BriefingSlots;
   notifications: boolean;
   temp_unit: 'C' | 'F';
+  timezone: string;
 }
 
 export interface BriefingFeed {
@@ -362,6 +363,7 @@ const DEFAULT_BRIEFING_CONFIG: BriefingConfig = {
   slots: { compilation: true, morning: true, midday: false, afternoon: false },
   notifications: true,
   temp_unit: 'C',
+  timezone: '',
 };
 
 export async function getBriefingConfig(): Promise<BriefingConfig> {