fix(admin): second review pass

Correctness
- /api/remote never recorded source.created, so URL, GitHub and connector
  sources had a source.deleted with no matching creation. All three creation
  paths now go through one _audit_source_created helper.
- The prompt-cache rate divided cached tokens by a whole bucket's prompt
  tokens. A bucket is a day and mixes calls whose provider reports a cache
  breakdown with calls whose provider does not, so filtering buckets in the
  client could not separate them and the rate was understated by however much
  traffic ran on a non-reporting provider. The denominator is now computed in
  SQL over the reporting rows.
- The outcome pill matched values nothing writes. Guardrails emit triggered /
  not_evaluated and the device feed emits dispatched; the map had blocked /
  denied / allowed, so a guardrail that fired rendered neutral grey -- the one
  signal the merged feed exists to surface. Fixtures were seeding the
  fictional values, so the tests passed on it too.
- Stream duration_ms timed the consumer. stream_token_usage is a generator,
  so start-to-exhaustion includes the agent loop's tool handling and the SSE
  client's pace; a slow browser recorded ~30s for a sub-second call. It now
  accumulates only the time spent inside next().

Safety
- Activity filters failed open: an unknown facet or unparseable timestamp was
  dropped, and no filter means every row, so a typo widened an audit view and
  on the export streamed the full history. Both are now a 400.
- The search term was interpolated into an ILIKE pattern, so "100%" matched
  everything and "q1_report" matched more than it should. Escaped.
- 0034 set actor_id NOT NULL with no default. A previous-release process
  inserting mid-rollout would raise, and in admin/routes.py that insert shares
  the request transaction, so a role grant beside it would roll back too.

Noise and dead code
- The per-user panel is a security panel: data-plane events file under the
  actor, so an active account's routine deletes pushed a denied login out of
  the 20-row window. It now excludes them; the Activity tab shows everything.
- device_audit_log had no created_at-leading index, so the merged feed
  sequentially scanned that branch every page (migration 0036).
- conversation.deleted_all no longer records when nothing was deleted, and
  agent.updated no longer records an empty field list.
- Dropped by_model from /admin/usage (no consumer; an extra aggregate per page
  load), the duplicate filter surface on AuthEventsRepository that nothing
  called, and the unreachable FLOW_LABELS.schedule entry.
- Type hints on record_event's conn and the remaining unannotated helpers.

docsgpt/api/user/conversations/routes.py

@@ -73,12 +73,15 @@ class DeleteAllConversations(Resource):
         try:
             with db_session() as conn:
                 deleted = ConversationsRepository(conn).delete_all_for_user(user_id)
-                record_event(
-                    conn,
-                    "conversation.deleted_all",
-                    actor=user_id,
-                    deleted=deleted,
-                )
+                # Nothing deleted is not an event; the endpoint is idempotent
+                # and a row here would render as a destructive action.
+                if deleted:
+                    record_event(
+                        conn,
+                        "conversation.deleted_all",
+                        actor=user_id,
+                        deleted=deleted,
+                    )
         except Exception as err:
             current_app.logger.error(
                 f"Error deleting all conversations: {err}", exc_info=True