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.

tests/api/test_admin_dashboard.py

@@ -238,12 +238,10 @@ class TestUsageAndAudit:
                 "generated_tokens": 3,
                 "cost": 0.25,
                 "cached_tokens": None,
+                "cache_eligible_prompt_tokens": 0,
             }
         ]
         usage.sum_tokens_in_range.return_value = 8
-        usage.tokens_by_model.return_value = [
-            {"model_id": "gpt-x", "tokens": 8, "cost": 0.25}
-        ]
         stats = Mock()
         stats.top_token_users.return_value = [
             {"user_id": "a", "tokens": 8, "cost": 0.25}
@@ -280,10 +278,20 @@ class TestUsageAndAudit:
             data = _body(client.get("/api/admin/usage"))
         assert data["total_cost"] == 0.25
         assert data["series"][0]["cost"] == 0.25
-        assert data["by_model"][0]["model_id"] == "gpt-x"
         assert data["latency"]["p95_ms"] == 400
         assert data["top_users"][0]["cost"] == 0.25
 
+    def test_usage_does_not_run_an_aggregate_nothing_reads(self, client):
+        """Per-model spend is served by the per-user endpoint and by group_by."""
+        usage, stats = self._usage_repos()
+        with _admin(
+            TokenUsageRepository=Mock(return_value=usage),
+            AdminStatsRepository=Mock(return_value=stats),
+        ):
+            data = _body(client.get("/api/admin/usage"))
+        assert "by_model" not in data
+        usage.tokens_by_model.assert_not_called()
+
     def test_usage_group_by_reaches_the_repository(self, client):
         usage, stats = self._usage_repos()
         with _admin(