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/apidoc/docs.go

@@ -22930,6 +22930,10 @@ const docTemplate = `{
                     "description": "AuditLogs controls how long audit log entries are retained.\nSet to 0 to disable (keep indefinitely).",
                     "type": "integer"
                 },
+                "boundary_logs": {
+                    "description": "BoundaryLogs controls how long boundary audit log entries are\nretained. Boundary logs record every HTTP request processed by\na Boundary confinement proxy. Set to 0 to disable automatic\ndeletion (keep indefinitely). Adjust to match your\norganization's regulatory requirements.",
+                    "type": "integer"
+                },
                 "connection_logs": {
                     "description": "ConnectionLogs controls how long connection log entries are retained.\nSet to 0 to disable (keep indefinitely).",
                     "type": "integer"

coderd/apidoc/swagger.json

@@ -21013,6 +21013,10 @@
 					"description": "AuditLogs controls how long audit log entries are retained.\nSet to 0 to disable (keep indefinitely).",
 					"type": "integer"
 				},
+				"boundary_logs": {
+					"description": "BoundaryLogs controls how long boundary audit log entries are\nretained. Boundary logs record every HTTP request processed by\na Boundary confinement proxy. Set to 0 to disable automatic\ndeletion (keep indefinitely). Adjust to match your\norganization's regulatory requirements.",
+					"type": "integer"
+				},
 				"connection_logs": {
 					"description": "ConnectionLogs controls how long connection log entries are retained.\nSet to 0 to disable (keep indefinitely).",
 					"type": "integer"