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>

backend/src/main/resources/db/migration/V16__notifications_and_preferences.sql

@@ -9,8 +9,10 @@ CREATE TABLE notifications (
     type          VARCHAR(32) NOT NULL,  -- 'REPLY' | 'MENTION'
     document_id   UUID,
     reference_id  UUID,                  -- commentId that triggered this notification
+    annotation_id UUID,
     read          BOOLEAN NOT NULL DEFAULT false,
-    created_at    TIMESTAMP NOT NULL DEFAULT now()
+    created_at    TIMESTAMP NOT NULL DEFAULT now(),
+    actor_name    VARCHAR(255)
 );
 
 CREATE INDEX idx_notifications_recipient ON notifications(recipient_id, read, created_at DESC);

backend/src/main/resources/db/migration/V18__add_actor_name_to_notifications.sql

@@ -1 +0,0 @@
-ALTER TABLE notifications ADD COLUMN actor_name VARCHAR(255);

backend/src/main/resources/db/migration/V19__add_annotation_id_to_notifications.sql

@@ -1 +0,0 @@
-ALTER TABLE notifications ADD COLUMN annotation_id UUID;