feat: deliver alert events through persistent outbox

backend/app/services/pinger.py

@@ -5,7 +5,6 @@ import time
 from collections import defaultdict
 from dataclasses import dataclass
 from datetime import datetime
-from typing import Awaitable, Callable
 
 from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
@@ -38,16 +37,6 @@ class Pinger:
     def __init__(self, runtime_settings: Settings | None = None):
         self._settings = runtime_settings or settings
         self._round_num = 0
-        self._on_state_change: (
-            Callable[[DeviceStateChange], Awaitable[None]] | None
-        ) = None
-
-    def on_state_change(
-        self,
-        callback: Callable[[DeviceStateChange], Awaitable[None]],
-    ) -> None:
-        """Register the legacy transition callback during scheduler migration."""
-        self._on_state_change = callback
 
     async def run_one_round(
         self,
@@ -128,11 +117,9 @@ class Pinger:
             if change is not None:
                 changes.append(change)
 
-        await db.commit()
-
-        if self._on_state_change is not None:
-            for change in changes:
-                await self._on_state_change(change)
+        # The scheduler records transition events and commits the complete
+        # probe/state/outbox unit.  A direct caller retains the same ownership.
+        await db.flush()
 
         elapsed = time.monotonic() - started_at
         logger.info(