fix: outbox KPI per-status legs, composite event-log indexes, AuditLog ingest FORCESEEK (#122, #123, #124)

#122: the Notification Outbox KPI snapshots ORed every status in one
aggregation, which scanned the clustered index once the table held ~1 M
Parked rows (0.7-2.6 s; Health dashboard up to 5 s). Each snapshot now runs
three status-equality legs that seek their own index (~0.12 s); a cold Health
load on the rig is 0.12-0.52 s.

#123: site_events event_type/severity indexes are now composites with
timestamp, so a filtered page is read in order and stops at the limit
(severity=Error over 1 GB: 2,287 -> 15 ms cold, 115 -> 1.6 ms warm). Costs
454 -> 530 B/row, ~14 % fewer rows under the same cap. The schema apply drops
the old indexes in place; the cap size model samples timestamp three times
and re-scores inside its band with unchanged constants.

#124: the set-based AuditLog insert's NOT EXISTS probe scanned the EventId
index and concurrent writers deadlocked on S/IX page locks. WITH (FORCESEEK):
4 writers 775 rows/s + 76 deadlocks in 30 s -> 5,214 rows/s + 0. New
concurrent-writer regression test fails 4/4 without the hint.

docs/components/SiteEventLogging.md

@@ -57,12 +57,16 @@ cmd.CommandText = """
         details TEXT
     );
     CREATE INDEX IF NOT EXISTS idx_events_timestamp ON site_events(timestamp);
-    CREATE INDEX IF NOT EXISTS idx_events_type ON site_events(event_type);
     CREATE INDEX IF NOT EXISTS idx_events_instance ON site_events(instance_id);
-    CREATE INDEX IF NOT EXISTS idx_events_severity ON site_events(severity);
+    DROP INDEX IF EXISTS idx_events_type;
+    DROP INDEX IF EXISTS idx_events_severity;
+    CREATE INDEX IF NOT EXISTS idx_events_type_timestamp ON site_events(event_type, timestamp);
+    CREATE INDEX IF NOT EXISTS idx_events_severity_timestamp ON site_events(severity, timestamp);
     """;
 ```
 
+The `event_type` and `severity` indexes are composites with `timestamp` (#123). Every query orders by `(timestamp, id)` and returns one page, so with a single-column index an equality filter read and sorted every matching row: `severity = Error` over a full 1 GB log took 2.3 s cold and 115 ms warm. The composite index returns rows already in timestamp order and stops at the page limit, in about 1 ms. The cost is about 69 bytes more index per row, which the storage-cap model charges, so a 1 GB cap now holds roughly 15 % fewer events. The two `DROP INDEX` statements migrate a pre-#123 file in place on its first boot; rebuilding the two indexes on a full 1 GB table takes seconds and happens once. Indexes are not part of LocalDb's replicated-table digest, so this change does not stop a mixed-version pair from syncing.
+
 Keyword search (`KeywordFilter`) runs as `LIKE '%…%' ESCAPE '\'` on `message` and `source`. A leading-wildcard `LIKE` cannot use a B-tree index, so keyword-only queries full-scan the table. All other filters (`event_type`, `severity`, `instance_id`, `timestamp`) are covered by the indexes above.
 
 ### Connection lock