Add asset proxy feature for resource URLs

Introduces asset proxy configuration and services to enable replacement of osu! resource URLs with custom domains. Updates API endpoints and caching services to process and rewrite resource URLs when asset proxy is enabled. Adds documentation and environment variables for asset proxy setup.

app/router/v2/beatmapset.py

@@ -14,6 +14,7 @@ from app.dependencies.user import get_client_user, get_current_user
 from app.fetcher import Fetcher
 from app.models.beatmap import SearchQueryModel
 from app.service.beatmap_download_service import BeatmapDownloadService
+from app.service.asset_proxy_helper import process_response_assets
 
 from .router import router
 
@@ -96,9 +97,12 @@ async def search_beatmapset(
     try:
         sets = await fetcher.search_beatmapset(query, cursor, redis)
         background_tasks.add_task(_save_to_db, sets)
+        
+        # 处理资源代理
+        processed_sets = await process_response_assets(sets, request)
+        return processed_sets
     except HTTPError as e:
         raise HTTPException(status_code=500, detail=str(e))
-    return sets
 
 
 @router.get(
@@ -110,13 +114,17 @@ async def search_beatmapset(
 )
 async def lookup_beatmapset(
     db: Database,
+    request: Request,
     beatmap_id: int = Query(description="谱面 ID"),
     current_user: User = Security(get_current_user, scopes=["public"]),
     fetcher: Fetcher = Depends(get_fetcher),
 ):
     beatmap = await Beatmap.get_or_fetch(db, fetcher, bid=beatmap_id)
     resp = await BeatmapsetResp.from_db(beatmap.beatmapset, session=db, user=current_user)
-    return resp
+    
+    # 处理资源代理
+    processed_resp = await process_response_assets(resp, request)
+    return processed_resp
 
 
 @router.get(