feat: purge boundary logs past retention (#24815)

Add a periodic purge job for `boundary_logs` rows past their retention
threshold, following the same pattern as the existing audit log and
connection log purge jobs in `dbpurge`.

Expose a `--boundary-log-retention` deployment flag (env
`CODER_BOUNDARY_LOG_RETENTION`, YAML `retention.boundary_logs`). Default
is `0` (keep indefinitely). When set to a positive duration, `purgeTick`
deletes rows where `captured_at` is older than the threshold in batches
of 10,000, matching other log purge operations. The `boundary_logs`
label is added to the `records_purged_total` Prometheus counter.

Also removes the random-UUID fallback for `OwnerID` in
`dbgen.BoundarySession`. The previous fallback generated a UUID that
could never satisfy the `boundary_sessions_owner_id_fkey` FK constraint,
masking test setup bugs. Callers must now provide a valid user ID or
accept NULL (the legitimate "user deleted" state).

coderd/database/dbmetrics/querymetrics.go

@@ -722,6 +722,14 @@ func (m queryMetricsStore) DeleteOldBoundaryLogs(ctx context.Context, arg databa
 	return r0, r1
 }
 
+func (m queryMetricsStore) DeleteOldBoundarySessions(ctx context.Context, arg database.DeleteOldBoundarySessionsParams) (int64, error) {
+	start := time.Now()
+	r0, r1 := m.s.DeleteOldBoundarySessions(ctx, arg)
+	m.queryLatencies.WithLabelValues("DeleteOldBoundarySessions").Observe(time.Since(start).Seconds())
+	m.queryCounts.WithLabelValues(httpmw.ExtractHTTPRoute(ctx), httpmw.ExtractHTTPMethod(ctx), "DeleteOldBoundarySessions").Inc()
+	return r0, r1
+}
+
 func (m queryMetricsStore) DeleteOldChatDebugRuns(ctx context.Context, arg database.DeleteOldChatDebugRunsParams) (int64, error) {
 	start := time.Now()
 	r0, r1 := m.s.DeleteOldChatDebugRuns(ctx, arg)