fix(restore): pause timer-based DB writers before swap (Postgres deadlock)

close_all_connections() only disposes the engine's connection pool —
  asyncio tasks like print_scheduler.run() and the smart-plug snapshot
  loop wake on their 30 s cadence and lazily reopen pool connections
  holding RowExclusiveLock on print_queue / smart_plug_energy_snapshots.
  The restore's DROP TABLE ... CASCADE pass needs AccessExclusiveLock on
  every public table, producing an AB/BA deadlock that rolls back the
  entire restore transaction.

  Reproduced 2026-06-09 restoring a native install's backup into a fresh
  Docker+Postgres deploy:
    asyncpg.exceptions.DeadlockDetectedError: deadlock detected
    Process X waits for AccessExclusiveLock on relation 109940
    Process Y waits for RowExclusiveLock on relation 110182

  Fix:
  - Layer 1: pause print_scheduler / smart_plug_manager /
    notification_service / background_dispatch via their existing stop
    affordances before close_all_connections(), with a 1.0 s sleep for
    in-flight loop iterations to release sessions. Restore handler
    already requires a container restart on success, so the paused
    services come back via the next lifespan startup.

  - Layer 2: prepend SET LOCAL lock_timeout = '10s' to the begin-block
    in _import_sqlite_to_postgres so any reactive writer (per-printer
    MQTT, hourly AMS history recorder) that slips through the pause
    window fails fast and visibly instead of producing a new deadlock.

backend/app/api/routes/settings.py

@@ -694,6 +694,15 @@ async def _import_sqlite_to_postgres(sqlite_path: Path, postgres_url: str):
                     table.constraints.discard(fk)
 
         async with pg_engine.begin() as conn:
+            # Cap how long DROP TABLE will wait for AccessExclusiveLock so
+            # any residual concurrent writer (per-printer MQTT clients
+            # writing reactively, an AMS history recorder firing on its
+            # hourly cadence) surfaces a fast `lock_timeout` error instead
+            # of blocking the restore for 30 s or producing a deadlock.
+            # SET LOCAL scopes to this transaction only; outside this
+            # restore path the global default (no timeout) applies.
+            await conn.execute(text("SET LOCAL lock_timeout = '10s'"))
+
             # Drop every existing table in the public schema with CASCADE
             # rather than `metadata.drop_all`. Two reasons:
             #   1. The user's live DB may carry orphan tables from removed
@@ -910,6 +919,35 @@ async def restore_backup(
             except Exception as e:
                 logger.warning("Failed to stop virtual printer: %s", e)
 
+            # 3b. Pause timer-based background services BEFORE the DB swap.
+            # close_all_connections() below only disposes the engine's pool,
+            # not the asyncio tasks that opened sessions from it. The print
+            # scheduler (30 s cadence), smart-plug snapshot loop (30 s),
+            # notification digest loop, and background dispatch worker all
+            # wake up and call async_session(), which lazily re-creates a
+            # pool connection holding RowExclusiveLock on print_queue /
+            # smart_plug_energy_snapshots / etc. The DROP TABLE CASCADE
+            # pass in the PostgreSQL restore path needs AccessExclusiveLock
+            # on every public table, producing an AB/BA deadlock and a
+            # full restore rollback. Successful restore already requires a
+            # container restart, so we don't restart the services here.
+            try:
+                from backend.app.services.background_dispatch import background_dispatch
+                from backend.app.services.notification_service import notification_service
+                from backend.app.services.print_scheduler import scheduler as print_scheduler
+                from backend.app.services.smart_plug_manager import smart_plug_manager
+
+                logger.info("Pausing background services for restore...")
+                print_scheduler.stop()
+                smart_plug_manager.stop_scheduler()
+                notification_service.stop_digest_scheduler()
+                await background_dispatch.stop()
+                # In-flight loop iterations need a moment to commit + release
+                # their DB sessions before we dispose() the engine pool.
+                await asyncio.sleep(1.0)
+            except Exception as e:
+                logger.warning("Could not cleanly pause background services: %s", e)
+
             # 4. Close current database connections
             logger.info("Closing database connections...")
             await close_all_connections()