feat(api): add internal-face break-glass world backup endpoint (§B4 Sync)

Add POST /api/v1/internal/servers/{name}/backup so the on-node break-glass
console can snapshot a stopped world while felis-api is alive. It goes through
the API (not direct-to-CRD like halt) because rendering the backup Job needs
deployment coordinates (FELIS_IMAGE, FELIS_BACKUP_PVC) only felis-api holds.

Service-token auth (no Principal); the middleware IS the authorization, since
the operator already has root on the node. Refactor the RWO stopped-gate,
optional-Backuper 503, async hand-off and audit+202 into a shared enqueueBackup
tail so the external (owner/admin) and internal (break-glass) faces cannot
diverge on the security-critical stopped-gate. The internal audit is attributed
to break-glass/internal so a console-initiated backup is distinguishable from an
owner self-service one.

internal/api/api.go

@@ -266,6 +266,11 @@ func (a *API) internalAPIRoutes() []apiRoute {
 		// Principal); the external face carries the start/status/finish the op drives.
 		{Method: "GET", Pattern: "/api/v1/internal/op-login/pending", h: a.handleOpLoginPending},
 		{Method: "POST", Pattern: "/api/v1/internal/op-login/{id}/approve", h: a.handleOpLoginApprove},
+
+		// Break-glass backup (spec §B4 "Sync"): the on-node console POSTs here to
+		// snapshot a stopped world while the API is alive. Service-token auth (no
+		// Principal); the shared enqueueBackup tail enforces the RWO stopped-gate.
+		{Method: "POST", Pattern: "/api/v1/internal/servers/{name}/backup", h: a.handleInternalBackup},
 	}
 }