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/views/SettingsView.vue

@@ -138,6 +138,10 @@ const addingFeed = ref(false);
 async function loadBriefingTab() {
   briefingConfig.value = await getBriefingConfig();
   briefingFeeds.value = await getBriefingFeeds();
+  // Auto-populate timezone from browser if not already stored.
+  if (!briefingConfig.value.timezone) {
+    briefingConfig.value.timezone = Intl.DateTimeFormat().resolvedOptions().timeZone;
+  }
 }
 
 async function geocodeLocation(key: 'home' | 'work') {
@@ -173,17 +177,6 @@ function toggleWorkDay(day: number) {
   days.sort();
 }
 
-/** Convert a UTC hour (0–23) to the browser's local time string, e.g. "6:00 am (UTC 4:00)" */
-function utcSlotToLocal(utcHour: number): string {
-  const now = new Date();
-  const utcMs = Date.UTC(now.getUTCFullYear(), now.getUTCMonth(), now.getUTCDate(), utcHour, 0, 0);
-  const local = new Date(utcMs);
-  const localStr = local.toLocaleTimeString([], { hour: 'numeric', minute: '2-digit' });
-  const utcStr = utcHour === 0 ? '12:00 am' : utcHour < 12
-    ? `${utcHour}:00 am`
-    : utcHour === 12 ? '12:00 pm' : `${utcHour - 12}:00 pm`;
-  return `${localStr} (UTC ${utcStr})`;
-}
 
 async function saveBriefingSettings() {
   briefingSaving.value = true;
@@ -1361,35 +1354,61 @@ function formatUserDate(iso: string): string {
         </div>
       </section>
 
+      <!-- Timezone -->
+      <section class="settings-section full-width">
+        <h2>Timezone</h2>
+        <p class="section-desc">
+          Briefing slots fire at the times below in this timezone.
+          Auto-detected from your browser — override if the server should use a different zone.
+        </p>
+        <div class="briefing-timezone-row">
+          <input
+            type="text"
+            class="input"
+            v-model="briefingConfig.timezone"
+            placeholder="e.g. America/New_York"
+            style="flex: 1"
+          />
+          <button
+            type="button"
+            class="btn-secondary"
+            @click="briefingConfig.timezone = Intl.DateTimeFormat().resolvedOptions().timeZone"
+          >Detect</button>
+        </div>
+        <p class="field-hint">
+          Use an
+          <a href="https://en.wikipedia.org/wiki/List_of_tz_database_time_zones" target="_blank" rel="noopener">IANA timezone name</a>
+          (e.g. <code>Europe/London</code>, <code>America/Chicago</code>).
+          Your browser reports: <strong>{{ Intl.DateTimeFormat().resolvedOptions().timeZone }}</strong>
+        </p>
+      </section>
+
       <!-- Slots -->
       <section class="settings-section full-width">
         <h2>Scheduled Slots</h2>
-        <p class="section-desc">
-          Each active slot posts an update into your briefing conversation.
-          Slots run on the server in UTC — your local equivalent is shown below.
-        </p>
+        <p class="section-desc">Each active slot posts an update into your briefing conversation at the listed local time.</p>
         <div class="briefing-slot-list">
           <div
-            v-for="[key, label, utcHour] in ([
-              ['compilation', 'Morning briefing', 4],
-              ['morning', 'Office check-in', 8],
-              ['midday', 'Midday update', 12],
-              ['afternoon', 'End of day', 16],
+            v-for="[key, label, localTime] in ([
+              ['compilation', 'Morning briefing', '4:00 am'],
+              ['morning', 'Office check-in', '8:00 am'],
+              ['midday', 'Midday update', '12:00 pm'],
+              ['afternoon', 'End of day', '4:00 pm'],
             ] as const)"
             :key="key"
             class="briefing-slot-row"
           >
             <div class="briefing-slot-info">
               <span class="briefing-slot-label">{{ label }}</span>
-              <span class="briefing-slot-time">{{ utcSlotToLocal(utcHour) }}</span>
+              <span class="briefing-slot-time">{{ localTime }}</span>
             </div>
             <label>
               <input type="checkbox" v-model="briefingConfig.slots[key]" />
             </label>
           </div>
         </div>
-        <p class="field-hint" style="margin-top: 0.5rem">
-          Your browser's local timezone: <strong>{{ Intl.DateTimeFormat().resolvedOptions().timeZone }}</strong>
+        <p v-if="briefingConfig.timezone" class="field-hint" style="margin-top: 0.5rem">
+          Firing in timezone: <strong>{{ briefingConfig.timezone }}</strong>
         </p>
       </section>
 
@@ -2731,6 +2750,12 @@ function formatUserDate(iso: string): string {
   flex-wrap: wrap;
   margin-top: 0.5rem;
 }
+.briefing-timezone-row {
+  display: flex;
+  gap: 0.5rem;
+  align-items: center;
+  margin-bottom: 0.5rem;
+}
 .briefing-unit-toggle {
   display: flex;
   gap: 0;