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/NotificationBell.svelte

@@ -1,7 +1,6 @@
 <script lang="ts">
 import { onMount, onDestroy } from 'svelte';
 import { goto } from '$app/navigation';
-import { PUBLIC_NOTIFICATION_POLL_MS } from '$env/static/public';
 import { m } from '$lib/paraglide/messages.js';
 
 type NotificationItem = {
@@ -16,15 +15,14 @@ type NotificationItem = {
 };
 
 let notifications: NotificationItem[] = $state([]);
-let unreadCount = $derived(notifications.filter((n) => !n.read).length);
+let unreadCount: number = $state(0);
 let open = $state(false);
 
 // DOM refs managed via attachments
 let bellButtonEl: HTMLButtonElement | null = null;
 let firstFocusableEl: HTMLButtonElement | null = null;
 
-const pollMs = Number(PUBLIC_NOTIFICATION_POLL_MS) || 60000;
-let intervalId: ReturnType<typeof setInterval>;
+let eventSource: EventSource | null = null;
 
 async function fetchNotifications() {
 	try {
@@ -38,6 +36,18 @@ async function fetchNotifications() {
 	}
 }
 
+async function fetchUnreadCount() {
+	try {
+		const res = await fetch('/api/notifications/unread-count');
+		if (res.ok) {
+			const data = await res.json();
+			unreadCount = data.count;
+		}
+	} catch (e) {
+		console.error('Failed to fetch unread count', e);
+	}
+}
+
 async function toggleDropdown() {
 	open = !open;
 	if (open) {
@@ -59,6 +69,7 @@ async function markRead(notification: NotificationItem) {
 		try {
 			await fetch(`/api/notifications/${notification.id}/read`, { method: 'PATCH' });
 			notification.read = true;
+			unreadCount = Math.max(0, unreadCount - 1);
 		} catch (e) {
 			console.error('Failed to mark notification as read', e);
 		}
@@ -76,6 +87,7 @@ async function markAllRead() {
 		for (const n of notifications) {
 			n.read = true;
 		}
+		unreadCount = 0;
 	} catch (e) {
 		console.error('Failed to mark all notifications as read', e);
 	}
@@ -133,11 +145,17 @@ function relativeTime(isoString: string): string {
 
 onMount(() => {
 	fetchNotifications();
-	intervalId = setInterval(fetchNotifications, pollMs);
+	fetchUnreadCount();
+	eventSource = new EventSource('/api/notifications/stream');
+	eventSource.addEventListener('notification', (e) => {
+		const notification = JSON.parse(e.data) as NotificationItem;
+		notifications = [notification, ...notifications];
+		if (!notification.read) unreadCount += 1;
+	});
 });
 
 onDestroy(() => {
-	clearInterval(intervalId);
+	eventSource?.close();
 });
 </script>
 
@@ -189,6 +207,7 @@ onDestroy(() => {
 	{#if open}
 		<div
 			role="dialog"
+			aria-modal="true"
 			aria-label={m.notification_bell_label()}
 			class="absolute right-0 z-50 mt-2 w-80 overflow-hidden rounded-sm border border-line bg-surface shadow-lg"
 		>