(feature) Add run completion notifications

Add email and webhook notification targets with delivery tracking, and send
notifications when backup runs reach a terminal status.

Expose notification target management in the Django UI and keep delivery
failures recorded without failing the backup worker.

src/pobsync_backend/backup_runner.py

@@ -17,6 +17,7 @@ from pobsync.commands.run_scheduled import (
 )
 from pobsync_backend.config_source import DjangoConfigSource
 from pobsync_backend.models import BackupRun, HostConfig
+from pobsync_backend.notifications import notify_backup_run_completed
 from pobsync_backend.retention import run_sql_retention_apply
 from pobsync_backend.snapshot_discovery import infer_snapshot_kind, upsert_snapshot_record
 
@@ -85,6 +86,7 @@ def execute_backup_run(
             "type": type(exc).__name__,
         }
         run.save(update_fields=["status", "ended_at", "result"])
+        notify_backup_run_completed(run)
         raise
 
     run.refresh_from_db()
@@ -151,6 +153,7 @@ def execute_backup_run(
             "result",
         ],
     )
+    notify_backup_run_completed(run)
     return run
 
 
@@ -277,6 +280,7 @@ def _reconcile_running_run(*, run: BackupRun, grace_seconds: int, stale_worker_s
             run.rsync_exit_code = exit_code or 255
             run.result = result
             run.save(update_fields=["status", "ended_at", "rsync_exit_code", "result"])
+            notify_backup_run_completed(run)
             return True
         if _running_rsync_process_missing(run=run, grace_seconds=grace_seconds):
             result.update(
@@ -301,6 +305,7 @@ def _reconcile_running_run(*, run: BackupRun, grace_seconds: int, stale_worker_s
             run.rsync_exit_code = exit_code or 255
             run.result = result
             run.save(update_fields=["status", "ended_at", "rsync_exit_code", "result"])
+            notify_backup_run_completed(run)
             return True
         if stale_worker:
             result.update(
@@ -318,6 +323,7 @@ def _reconcile_running_run(*, run: BackupRun, grace_seconds: int, stale_worker_s
             run.ended_at = timezone.now()
             run.result = result
             run.save(update_fields=["status", "ended_at", "result"])
+            notify_backup_run_completed(run)
             return True
         return False
 
@@ -353,6 +359,7 @@ def _reconcile_running_run(*, run: BackupRun, grace_seconds: int, stale_worker_s
     run.rsync_exit_code = exit_code
     run.result = result
     run.save(update_fields=["status", "ended_at", "rsync_exit_code", "result"])
+    notify_backup_run_completed(run)
     return True