feat(#71,#72,#73): SSE push notifications, mention chips, deep-link fixes

- Add SseEmitterRegistry (ConcurrentHashMap, one emitter per user)
- Add GET /api/notifications/stream SSE endpoint and unread-count endpoint
- Push SSE event on every notifyReply / notifyMentions via saveAndPush()
- Collapse V18/V19 migrations into V16 (actor_name + annotation_id upfront)
- Add @Schema(requiredMode=REQUIRED) to NotificationDTO required fields
- Switch NotificationBell from polling to EventSource; seed unread count on open
- Fix MentionEditor: replace setTimeout with await tick(); div role=option
- Add aria-modal=true to NotificationBell dialog
- Tests: SseEmitterRegistryTest (3), NotificationServiceTest (+2), NotificationControllerTest (+5)

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

frontend/src/lib/components/user/UserGroupsSection.svelte

@@ -6,6 +6,8 @@ let {
 	groups: { id: string; name: string }[];
 	selectedGroupIds?: string[];
 } = $props();
+
+let selected = $derived([...selectedGroupIds]);
 </script>
 
 <div class="flex flex-wrap gap-3">
@@ -15,7 +17,7 @@ let {
 				type="checkbox"
 				name="groupIds"
 				value={group.id}
-				checked={selectedGroupIds.includes(group.id)}
+				bind:group={selected}
 				class="rounded border-line text-ink focus:ring-accent"
 			/>
 			{group.name}