feat: v6.0.0 PWA offline support

- manifest + icones, service worker (precache, network-first, Background Sync)

- module client FlowOffline (IndexedDB, queue, delta, flush) + hook editeur

- endpoints /api/v2/sync/{delta,batch,status} + moteur de sync (conflits LWW/orpheline/copie offline)

- migrations offline_sync_queue + sync_version (triggers)

- UI offline (banner, badge sync, toasts, icone dirty) + doc /help

- tests pytest (sync, migrations, SW, offline) + E2E Playwright; bump 6.0.0

app/migrations.py

@@ -630,3 +630,82 @@ def _migration_synced_blocks(conn: sqlite3.Connection) -> None:
     conn.execute(
         "CREATE INDEX IF NOT EXISTS idx_psb_synced ON page_synced_blocks(synced_block_id)"
     )
+
+
+@register(16, "v6.0.0: offline sync queue")
+def _migration_offline_sync_queue(conn: sqlite3.Connection) -> None:
+    """v6.0.0 — PWA offline support.
+
+    ``offline_sync_queue`` persists server-side the mutations received from
+    offline clients (``/api/v2/sync/batch``) so work is not lost and can be
+    audited/replayed per device.
+    """
+    conn.execute(
+        """CREATE TABLE IF NOT EXISTS offline_sync_queue (
+            id INTEGER PRIMARY KEY AUTOINCREMENT,
+            user_id INTEGER NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+            device_id TEXT NOT NULL,
+            type TEXT NOT NULL,
+            -- 'page_create', 'page_update', 'page_delete', 'page_move',
+            -- 'collection_create', 'collection_update', 'collection_delete'
+            payload TEXT NOT NULL,
+            client_timestamp REAL NOT NULL,
+            server_version INTEGER DEFAULT 0,
+            status TEXT NOT NULL DEFAULT 'pending',
+            -- 'pending', 'syncing', 'synced', 'failed'
+            retries INTEGER NOT NULL DEFAULT 0,
+            error TEXT,
+            created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
+        )"""
+    )
+    conn.execute(
+        "CREATE INDEX IF NOT EXISTS idx_syncqueue_user ON offline_sync_queue(user_id, status)"
+    )
+    conn.execute(
+        "CREATE INDEX IF NOT EXISTS idx_syncqueue_device ON offline_sync_queue(device_id, status)"
+    )
+
+
+def _add_sync_version(conn: sqlite3.Connection, table: str) -> None:
+    """Add ``sync_version`` to ``table`` if it is not already present."""
+    cols = {row[1] for row in conn.execute(f"PRAGMA table_info({table})").fetchall()}
+    if "sync_version" not in cols:
+        conn.execute(f"ALTER TABLE {table} ADD COLUMN sync_version INTEGER NOT NULL DEFAULT 1")
+
+
+@register(17, "v6.0.0: sync_version columns")
+def _migration_sync_version_columns(conn: sqlite3.Connection) -> None:
+    """v6.0.0 — optimistic-concurrency version counters for offline sync.
+
+    Every write on a page / collection increments its ``sync_version`` so a
+    reconnecting client can detect edit-edit conflicts via version mismatch.
+    A ``BEFORE UPDATE`` trigger performs the increment automatically on every
+    write path (no need to patch dozens of ``UPDATE`` call-sites).
+    """
+    for table in ("pages", "collection_pages", "collections"):
+        _add_sync_version(conn, table)
+
+    # AFTER UPDATE + inner UPDATE: bumps sync_version on every write path.
+    # `recursive_triggers` is OFF by default, so the inner UPDATE never
+    # re-fires the trigger (no infinite loop), incl. the page FTS triggers.
+    conn.execute(
+        """CREATE TRIGGER IF NOT EXISTS pages_sync_version_bu
+           AFTER UPDATE ON pages
+           FOR EACH ROW BEGIN
+             UPDATE pages SET sync_version = sync_version + 1 WHERE id = NEW.id;
+           END"""
+    )
+    conn.execute(
+        """CREATE TRIGGER IF NOT EXISTS collection_pages_sync_version_bu
+           AFTER UPDATE ON collection_pages
+           FOR EACH ROW BEGIN
+             UPDATE collection_pages SET sync_version = sync_version + 1 WHERE id = NEW.id;
+           END"""
+    )
+    conn.execute(
+        """CREATE TRIGGER IF NOT EXISTS collections_sync_version_bu
+           AFTER UPDATE ON collections
+           FOR EACH ROW BEGIN
+             UPDATE collections SET sync_version = sync_version + 1 WHERE id = NEW.id;
+           END"""
+    )