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).

site/src/api/typesGenerated.ts

@@ -7244,6 +7244,14 @@ export interface RetentionConfig {
 	 * Defaults to 7 days to preserve existing behavior.
 	 */
 	readonly workspace_agent_logs: number;
+	/**
+	 * BoundaryLogs controls how long boundary audit log entries are
+	 * retained. Boundary logs record every HTTP request processed by
+	 * a Boundary confinement proxy. Set to 0 to disable automatic
+	 * deletion (keep indefinitely). Adjust to match your
+	 * organization's regulatory requirements.
+	 */
+	readonly boundary_logs: number;
 }
 
 // From codersdk/roles.go