feat: v5.14.0 Synced blocks - block created once, edited everywhere

- Table synced_blocks (source of truth) + page_synced_blocks (references)
- Migration 15 + service app/services/synced_blocks.py
- API endpoints: CRUD synced blocks, add/remove page references
- Editor: /synced slash command, synced block rendering with badge
- Realtime: _propagate_synced broadcasts updates to all referencing rooms
- Export: synced blocks resolved in Markdown/HTML/PDF
- 18 tests in tests/test_v514_synced_blocks.py

Co-authored-by: FlowDeck <[email protected]>

app/templates/_page_editor_realtime.html

@@ -384,13 +384,19 @@ window.__fdRT = (function () {
         peers = peers.filter(p => p.id !== m.peer.id).concat([m.peer]);
         renderPresence();
       }
-    } else if (m.t === 'peer_leave') {
-      peers = peers.filter(p => p.id !== m.id);
-      delete remoteCursors[m.id];
-      renderPresence();
-      scheduleDraw();
-    }
-  }
+     } else if (m.t === 'peer_leave') {
+       peers = peers.filter(p => p.id !== m.id);
+       delete remoteCursors[m.id];
+       renderPresence();
+       scheduleDraw();
+     } else if (m.t === 'synced_update') {
+       // A synced block was updated — re-sync the whole page
+       if (m.synced_id) {
+         needSync = true;
+         send({ t: 'sync_req' });
+       }
+     }
+   }
 
   /* ── connexion WS + fallback polling ── */
   function startPolling() {