feat(redis): refactor Redis configuration to use multiple logical databases

- Updated default REDIS_URL to remove explicit /0 suffix
- Added dedicated Redis clients:
  - db0: general cache (redis_client)
  - db1: message cache (redis_message_client)
  - db2: binary storage (redis_binary_client)
  - db3: rate limiting (redis_rate_limit_client)
- Updated configuration, Docker files, and main startup lifecycle accordingly
- Replaced `get_redis()` usage in notification server with `redis_message_client`

.env.example

@@ -6,7 +6,7 @@ MYSQL_DATABASE="osu_api"
 MYSQL_USER="osu_api"
 MYSQL_PASSWORD="password"
 MYSQL_ROOT_PASSWORD="password"
-REDIS_URL="redis://127.0.0.1:6379/0"
+REDIS_URL="redis://127.0.0.1:6379"
 
 # JWT Settings
 # Use `openssl rand -hex 32` to generate a secure key