feat(config): add server config + can disable message-bus

- server now provide it's config via /api/config (possibility to feature flag)
- client use /api/config to enable / disable some features
- capability to disable the message bus, somme OPS may not want this feature and
  consume persistent connections from server (websocket):
  OXICLOUD_MESSAGEBUS_ENABLE (true by default)

tests/api/rt_bus_check.sh

@@ -297,7 +297,7 @@ log "S4 OK"
 
 # ── Scenario 5 — Server-initiated keepalive ─────────────────────────────────
 # Verifies the WS handler sends RFC 6455 Ping control frames on the
-# `OXICLOUD_RT_WS_KEEPALIVE_SECONDS` cadence (1 s in tests/common/server.env).
+# `OXICLOUD_MESSAGEBUS_KEEPALIVE_SECONDS` cadence (1 s in tests/common/server.env).
 # Two invariants:
 #   (a) idling on a live subscription surfaces multiple Ping frames — the
 #       keepalive interval genuinely fires, not just at connect and never again.

tests/common/server.env

@@ -40,7 +40,7 @@ OXICLOUD_NEXTCLOUD_ENABLED=true
 # in `tests/api/rt_bus_check.sh` can observe multiple keepalive frames
 # arriving within a bounded (few-seconds) wall-clock. Only observed
 # by `rt_ws_handler`, which reads it at each WS connect time.
-OXICLOUD_RT_WS_KEEPALIVE_SECONDS=1
+OXICLOUD_MESSAGEBUS_KEEPALIVE_SECONDS=1
 
 # Multi-entry storage config — see docs/plan/storage-multi-entry.md.
 # `local_main` is FIRST so the boot fallback picks it when no active