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/audit.py

@@ -25,6 +25,7 @@ import logging
 from typing import Any, Optional
 
 from flask import has_request_context, request
+from sqlalchemy import Connection
 
 from docsgpt.storage.db.repositories.auth_events import AuthEventsRepository
 
@@ -32,7 +33,7 @@ logger = logging.getLogger(__name__)
 
 
 def record_event(
-    conn,
+    conn: Connection,
     event: str,
     *,
     actor: Optional[str],
@@ -42,8 +43,9 @@ def record_event(
     """Append one audit event, best-effort, inside the caller's transaction.
 
     Args:
-        conn: Open database connection, inside the action's transaction so the
-            audit row commits atomically with the change it describes.
+        conn: Open SQLAlchemy ``Connection`` -- not a Session -- inside the
+            action's transaction, so the audit row commits atomically with the
+            change it describes and ``begin_nested`` gives it a real SAVEPOINT.
         event: Dotted event name (``source.deleted``, ``agent.created``).
         actor: Who performed the action; ``"unknown"`` when unauthenticated.
         target: The user acted upon, when the event is about a user. Data-plane