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/querier.go

@@ -175,6 +175,9 @@ type sqlcQuerier interface {
 	// Deletes boundary logs older than the given time, bounded by a row limit
 	// to avoid long-running transactions.
 	DeleteOldBoundaryLogs(ctx context.Context, arg DeleteOldBoundaryLogsParams) (int64, error)
+	// Deletes boundary sessions that have aged past retention and no longer
+	// have any associated logs.
+	DeleteOldBoundarySessions(ctx context.Context, arg DeleteOldBoundarySessionsParams) (int64, error)
 	// updated_at is the retention clock, so the window starts after the run
 	// stops being written to.
 	// Intentionally no finished_at IS NOT NULL guard: abandoned in-flight rows