feat: per-domain backend routing

Each allowed_domains entry can now carry its own backend {host, port}.
That lets one proxy on port 25565 serve multiple MC servers, picking
the upstream from the domain the client typed.

- proxy/main.py: ProxyState.backend_for(domain) → tuple|None,
  honors per-domain backend first, falls back to top-level backend.
  handle_client uses backend_for(); blocked / disabled domains
  return None (and still get a Login Disconnect on join attempts).
- api/routes/{config,domains}.py: DomainBackend model + optional
  backend field on create/patch. PATCH supports clear_backend=true
  to drop a per-domain override and revert to default.
- frontend/Domains.jsx: full rewrite — new-domain form has host/port
  inputs, table shows each row's effective backend, inline edit +
  reset button per row.
- frontend/Settings.jsx: backend section relabeled "기본 백엔드 (fallback)"
- README updated with multi-server example config.

proxy/main.py

@@ -129,9 +129,31 @@ class ProxyState:
         return cfg_mod.allowed_domain_set(self.cfg)
 
     def backend(self) -> tuple[str, int]:
+        """기본 백엔드 (도메인 entry 에 backend 가 없을 때 fallback)."""
         b = self.cfg["backend"]
         return b["host"], int(b["port"])
 
+    def backend_for(self, domain: str) -> tuple[str, int] | None:
+        """주어진 도메인이 활성 화이트리스트에 있으면 라우팅 대상을 돌려준다.
+
+        도메인 entry 에 `backend.host`/`backend.port` 가 있으면 그 값을 우선,
+        없으면 top-level `backend` 로 fallback. 도메인이 비활성이거나 없으면
+        None.
+        """
+        d = domain.lower().strip()
+        for entry in self.cfg.get("allowed_domains", []):
+            if entry["domain"].lower().strip() != d:
+                continue
+            if not entry.get("enabled", True):
+                return None
+            be = entry.get("backend") or {}
+            host = (be.get("host") or "").strip()
+            port = be.get("port")
+            if host and port:
+                return host, int(port)
+            return self.backend()
+        return None
+
     def enabled(self) -> bool:
         return bool(self.cfg.get("proxy", {}).get("enabled", True))
 
@@ -212,8 +234,8 @@ async def handle_client(
         return
 
     domain = hs.server_address.lower().strip()
-    allowed = state.allowed()
-    if domain not in allowed:
+    target = state.backend_for(domain)
+    if target is None:
         log_event(client_ip, domain, hs.next_state, "blocked", "domain not allowed")
         log.info(
             "BLOCK %s domain=%r next_state=%d", client_ip, domain, hs.next_state
@@ -230,7 +252,7 @@ async def handle_client(
         client_writer.close()
         return
 
-    backend_host, backend_port = state.backend()
+    backend_host, backend_port = target
     try:
         backend_reader, backend_writer = await asyncio.wait_for(
             asyncio.open_connection(backend_host, backend_port), timeout=5