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/migrations.py

@@ -588,3 +588,45 @@ def _migration_fix_llm_api_bases(conn: sqlite3.Connection) -> None:
                     f"UPDATE {table} SET api_base='' WHERE provider=? AND api_base=?",
                     (provider, value),
                 )
+
+
+@register(15, "v5.14.0: synced blocks")
+def _migration_synced_blocks(conn: sqlite3.Connection) -> None:
+    """v5.14.0 — Synced blocks: a block created once, displayed &
+    edited across multiple pages.
+
+    ``synced_blocks`` — source-of-truth content for synced blocks.
+    ``page_blocks``          — per-page reference to a synced block
+    (so each page can independently decide to use/unsync).
+    """
+    conn.execute(
+        """CREATE TABLE IF NOT EXISTS synced_blocks (
+            id INTEGER PRIMARY KEY AUTOINCREMENT,
+            title TEXT NOT NULL DEFAULT '',
+            content TEXT NOT NULL DEFAULT '[]',
+            created_by INTEGER REFERENCES users(id),
+            workspace TEXT NOT NULL DEFAULT '',
+            created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
+            updated_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
+        )"""
+    )
+    conn.execute(
+        "CREATE INDEX IF NOT EXISTS idx_synced_blocks_ws ON synced_blocks(workspace)"
+    )
+
+    conn.execute(
+        """CREATE TABLE IF NOT EXISTS page_synced_blocks (
+            id INTEGER PRIMARY KEY AUTOINCREMENT,
+            page_id INTEGER NOT NULL REFERENCES pages(id) ON DELETE CASCADE,
+            synced_block_id INTEGER NOT NULL REFERENCES synced_blocks(id) ON DELETE CASCADE,
+            block_index INTEGER NOT NULL DEFAULT 0,
+            created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
+            UNIQUE(page_id, synced_block_id)
+        )"""
+    )
+    conn.execute(
+        "CREATE INDEX IF NOT EXISTS idx_psb_page ON page_synced_blocks(page_id)"
+    )
+    conn.execute(
+        "CREATE INDEX IF NOT EXISTS idx_psb_synced ON page_synced_blocks(synced_block_id)"
+    )