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).

cli/testdata/coder_server_--help.golden

@@ -878,6 +878,12 @@ that data type.
           indefinitely). We advise keeping audit logs for at least a year, and
           in accordance with your compliance requirements.
 
+      --boundary-log-retention duration, $CODER_BOUNDARY_LOG_RETENTION (default: 0)
+          How long boundary audit log entries are retained. Boundary logs record
+          HTTP requests processed by a Boundary confinement proxy. Set to 0 to
+          disable automatic deletion (keep indefinitely). Adjust to match your
+          organization's regulatory requirements.
+
       --connection-logs-retention duration, $CODER_CONNECTION_LOGS_RETENTION (default: 0)
           How long connection log entries are retained. Set to 0 to disable
           (keep indefinitely).