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/database/lazer_user.py

@@ -89,7 +89,7 @@ class UserBase(UTCBaseModel, SQLModel):
     is_restricted: bool = False
     # blocks
     cover: UserProfileCover = Field(
-        default=UserProfileCover(url="https://assets.ppy.sh/user-profile-covers/default.jpeg"),
+        default=UserProfileCover(url=""),
         sa_column=Column(JSON),
     )
     beatmap_playcounts_count: int = 0
@@ -292,9 +292,9 @@ class UserResp(UserBase):
         redis = get_redis()
         u.is_online = await redis.exists(f"metadata:online:{obj.id}")
         u.cover_url = (
-            obj.cover.get("url", "https://assets.ppy.sh/user-profile-covers/default.jpeg")
+            obj.cover.get("url", "")
             if obj.cover
-            else "https://assets.ppy.sh/user-profile-covers/default.jpeg"
+            else ""
         )
 
         if "friends" in include: