feat: add SCGI connection pooling with bb8

- Implement ScgiConnectionManager with bb8 for Unix socket pooling
- Create connection pool with max 10 connections, min 5 idle
- Update RtorrentClient to use pooled connections instead of per-request connections
- Add ScgiPool to AppState for shared access across handlers
- Update all handlers to use pooled client
- Reduces file descriptor pressure and connection latency

backend/Cargo.toml

@@ -43,3 +43,5 @@ time = { version = "0.3.47", features = ["serde", "formatting", "parsing"] }
 tower_governor = "0.8.0"
 governor = "0.10.4"
 strum = { version = "0.25", features = ["derive", "strum_macros"] }
+bb8 = "0.8"
+async-trait = "0.1"