fix(auditlog)+fix(siteeventlog): complete audit write tasks only after commit; run event-log retention on standby (arch-review 2026-08-19 P4-2 + P4-4)

P4-2: SqliteAuditWriter.FlushBatch completed each pending row's Task
(TrySetResult) inside the per-row insert loop, before transaction.Commit()
ran. A later failure in the same batch (another row's insert throwing, or
the commit itself failing) rolled the whole transaction back, but the
earlier rows' Tasks had already completed successfully -- the caller and
FallbackAuditWriter (whose ring-buffer fallback exists precisely for this
outage) both saw false success while the rows were actually gone from the
database; the outer catch's TrySetException became a silent no-op against
an already-completed TaskCompletionSource. Every row's Completion is now
deferred and only resolved (TrySetResult for the whole batch, or
TrySetException for the whole batch) after transaction.Commit() succeeds
or the transaction rolls back, mirroring SiteEventLogger.FlushBatch.

P4-4: EventLogPurgeService gated the daily site_events purge on the
active-node check, early-exiting on the standby, on the stale premise that
"the standby receives no writes". DataConnectionActor runs on both site
nodes and writes site_events regardless of active/standby status (register
rows 89/90/97), so an unreplicated standby (the rig's site-b/site-c
posture) grew its site_events file forever with no retention and no cap.
The gate is removed; every node now purges its own file every tick. Safe
on a replicated pair because the retention/cap deletes are CDC-captured
and converge under LWW regardless of which node issues them; required on
an unreplicated pair because each node's file is independent. The
active-node check is retained only to annotate the purge tick's log line.

Tests: new SqliteAuditWriterCommitFailureTests forces a genuine
non-swallowed mid-batch INSERT failure via a real SQLite trigger + custom
scalar function on the writer's own connection (reflection seam), proving
(a) a row whose own insert succeeded still faults when a later row in the
same batch fails, and (b) FallbackAuditWriter rings every faulted row and
increments the failure counter for each -- both assertions fail against
the pre-fix code (proof-of-failure verified by temporarily reverting the
fix). EventLogPurgeServiceTests' standby case is inverted to assert the
purge now runs (not skips) on a standby node.

docs/requirements/Component-SiteEventLogging.md

@@ -77,8 +77,11 @@ Each event entry contains:
 ## Storage
 
 - Events are stored in the **consolidated site LocalDb** SQLite file (`site_events` table;
-  see CLAUDE.md → Consolidated site database). Only the **active node** generates and stores
-  events (a standby runs no scripts, subsystems, or deployments to log).
+  see CLAUDE.md → Consolidated site database). The active node generates most events (a
+  standby runs no scripts, subsystems, or deployments to log), but DataConnectionActor
+  runs on **both** site nodes and writes its own `site_events` rows regardless of
+  active/standby status (register rows 89/90/97) — see *Retention* below for the
+  purge consequence.
 - **`site_events` IS replicated** (`SiteLocalDbSetup.ReplicatedTables`) on a site pair with
   replication configured — this reverses the component's pre-LocalDb-Phase-1 behavior, and the
   reason is **failover history continuity, not either-node queries**: the query handler
@@ -92,7 +95,7 @@ Each event entry contains:
   On a site pair running **without** replication configured (by deliberate choice, e.g. the
   rig's site-b/site-c), the log stays node-local and a failover does start it fresh — the
   documented trade of not configuring a peer, unchanged by this policy.
-- **Retention**: 30 days. A **daily background job** runs on the active node and deletes all events older than 30 days. Hard delete — no archival. Deletes are sliced into bounded 1000-row batches per DELETE statement rather than one unbounded statement, so a large expired backlog does not hold the write lock (and every concurrent recorder flush) for the duration of the purge.
+- **Retention**: 30 days. A **daily background job** runs on **every** site node (arch-review 2026-08-19 P4-4 — previously active-node-only, but the standby writes its own `site_events` rows too, so an unreplicated standby's file grew unbounded under the old active-only gate) and deletes that node's own events older than 30 days. Hard delete — no archival. Deletes are sliced into bounded 1000-row batches per DELETE statement rather than one unbounded statement, so a large expired backlog does not hold the write lock (and every concurrent recorder flush) for the duration of the purge.
 - **Storage cap**: A configurable maximum size (default: 1 GB) is enforced. If the cap is reached
   before the 30-day retention window, the oldest events are purged first. This prevents disk
   exhaustion from alarm storms, script failure loops, or connection flapping.