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.

api/routes/config.py

@@ -19,10 +19,16 @@ class BackendConfig(BaseModel):
     port: int = Field(ge=1, le=65535)
 
 
+class DomainBackend(BaseModel):
+    host: str
+    port: int = Field(ge=1, le=65535)
+
+
 class DomainEntry(BaseModel):
     domain: str
     enabled: bool = True
     note: str = ""
+    backend: DomainBackend | None = None  # 없으면 top-level backend 로 fallback
 
 
 class FullConfig(BaseModel):