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/domains.py

@@ -2,22 +2,32 @@
 from __future__ import annotations
 
 from fastapi import APIRouter, HTTPException, Response
-from pydantic import BaseModel
+from pydantic import BaseModel, Field
 
 from config_io import load_config, save_config
 
 router = APIRouter()
 
 
+class DomainBackend(BaseModel):
+    host: str
+    port: int = Field(ge=1, le=65535)
+
+
 class DomainCreate(BaseModel):
     domain: str
     enabled: bool = True
     note: str = ""
+    backend: DomainBackend | None = None  # 비우면 기본 backend 사용
 
 
 class DomainPatch(BaseModel):
     enabled: bool | None = None
     note: str | None = None
+    # 명시적으로 None 을 보내려면 클라이언트가 "backend": null 을 보내야 한다.
+    # 빠뜨리면 기존 값 유지.
+    backend: DomainBackend | None = None
+    clear_backend: bool = False  # True 면 기존 backend 삭제 → 기본값으로 복귀
 
 
 @router.get("/domains")
@@ -34,7 +44,9 @@ def add_domain(body: DomainCreate) -> dict:
         raise HTTPException(status_code=400, detail="domain required")
     if any(d["domain"].lower() == name for d in domains):
         raise HTTPException(status_code=409, detail="domain already exists")
-    entry = {"domain": name, "enabled": body.enabled, "note": body.note}
+    entry: dict = {"domain": name, "enabled": body.enabled, "note": body.note}
+    if body.backend is not None:
+        entry["backend"] = body.backend.model_dump()
     domains.append(entry)
     save_config(cfg)
     return entry
@@ -64,6 +76,10 @@ def patch_domain(domain: str, body: DomainPatch) -> dict:
                 d["enabled"] = body.enabled
             if body.note is not None:
                 d["note"] = body.note
+            if body.clear_backend:
+                d.pop("backend", None)
+            elif body.backend is not None:
+                d["backend"] = body.backend.model_dump()
             save_config(cfg)
             return d
     raise HTTPException(status_code=404, detail="domain not found")