(release) Add purged snapshot audit overview

Record snapshot purge history whenever retention or incomplete cleanup removes
snapshot directories and SQL records. Store the purge reason, original kind,
path, action source, and triggering operator so manual, scheduled, CLI, and
incomplete cleanup actions remain auditable after the original snapshot record
is deleted.

Add a staff-only Purged Snapshots page with host/action filters and register
the audit model in Django admin.

Refs #16
Refs #8

src/pobsync_backend/retention.py

@@ -12,7 +12,7 @@ from pobsync.paths import PobsyncPaths
 from pobsync.retention import Snapshot, apply_base_protection, build_retention_plan
 from pobsync.util import sanitize_host
 
-from .models import HostConfig, SnapshotRecord
+from .models import HostConfig, PurgedSnapshot, SnapshotRecord
 
 
 def run_sql_retention_plan(*, host: str, kind: str, protect_bases: bool) -> dict[str, Any]:
@@ -65,6 +65,8 @@ def run_sql_retention_apply(
     protect_bases: bool,
     yes: bool,
     max_delete: int,
+    action: str = PurgedSnapshot.Action.MANUAL,
+    triggered_by: str = "",
     acquire_lock: bool = True,
 ) -> dict[str, Any]:
     host = sanitize_host(host)
@@ -101,6 +103,7 @@ def run_sql_retention_apply(
                 raise ConfigError(f"Refusing to delete unsupported snapshot kind: {snap_kind!r}")
 
             path = _snapshot_delete_path(path=Path(snap_path), dirname=dirname)
+            reason = str(item.get("reason") or "outside retention policy")
             if not path.exists():
                 actions.append(f"skip missing {snap_kind}/{dirname}")
                 continue
@@ -108,9 +111,19 @@ def run_sql_retention_apply(
                 raise ConfigError(f"Refusing to delete non-directory path: {path}")
 
             _remove_snapshot_tree(path)
+            _record_purged_snapshot(
+                host_config=_enabled_host_config(host),
+                kind=snap_kind,
+                dirname=dirname,
+                path=path,
+                reason=reason,
+                action=action,
+                triggered_by=triggered_by,
+                metadata={"source": "retention", "protect_bases": bool(protect_bases), "retention_kind": kind},
+            )
             SnapshotRecord.objects.filter(host__host=host, kind=snap_kind, dirname=dirname).delete()
             actions.append(f"deleted {snap_kind} {dirname}")
-            deleted.append({"dirname": dirname, "kind": snap_kind, "path": str(path)})
+            deleted.append({"dirname": dirname, "kind": snap_kind, "path": str(path), "reason": reason})
 
         return {
             "ok": True,
@@ -137,6 +150,7 @@ def run_incomplete_cleanup(
     host: str,
     yes: bool,
     max_delete: int,
+    triggered_by: str = "",
     acquire_lock: bool = True,
 ) -> dict[str, Any]:
     host = sanitize_host(host)
@@ -177,6 +191,16 @@ def run_incomplete_cleanup(
                 _remove_snapshot_tree(path)
                 actions.append(f"deleted incomplete {dirname}")
 
+            _record_purged_snapshot(
+                host_config=host_config,
+                kind=SnapshotRecord.Kind.INCOMPLETE,
+                dirname=dirname,
+                path=path,
+                reason="manual incomplete cleanup",
+                action=PurgedSnapshot.Action.INCOMPLETE_CLEANUP,
+                triggered_by=triggered_by,
+                metadata={"source": "incomplete_cleanup"},
+            )
             SnapshotRecord.objects.filter(
                 host__host=host,
                 kind=SnapshotRecord.Kind.INCOMPLETE,
@@ -282,6 +306,30 @@ def _snapshot_delete_path(*, path: Path, dirname: str) -> Path:
     return path
 
 
+def _record_purged_snapshot(
+    *,
+    host_config: HostConfig,
+    kind: str,
+    dirname: str,
+    path: Path,
+    reason: str,
+    action: str,
+    triggered_by: str,
+    metadata: dict[str, Any],
+) -> None:
+    PurgedSnapshot.objects.create(
+        host=host_config,
+        host_name=host_config.host,
+        kind=kind,
+        dirname=dirname,
+        path=str(path),
+        reason=reason,
+        action=action,
+        triggered_by=triggered_by,
+        metadata=metadata,
+    )
+
+
 def _validate_incomplete_delete_path(*, host: str, path: Path, dirname: str) -> None:
     path_parts = path.parts
     if path.name != dirname or ".incomplete" not in path_parts or host not in path_parts: