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

Cargo.lock

@@ -292,9 +292,11 @@ name = "backend"
 version = "0.1.0"
 dependencies = [
  "anyhow",
+ "async-trait",
  "axum",
  "axum-extra",
  "base64 0.22.1",
+ "bb8",
  "bcrypt",
  "bytes",
  "clap",
@@ -356,6 +358,18 @@ version = "1.8.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "2af50177e190e07a26ab74f8b1efbfe2ef87da2116221318cb1c2e82baf7de06"
 
+[[package]]
+name = "bb8"
+version = "0.8.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d89aabfae550a5c44b43ab941844ffcd2e993cb6900b342debf59e9ea74acdb8"
+dependencies = [
+ "async-trait",
+ "futures-util",
+ "parking_lot",
+ "tokio",
+]
+
 [[package]]
 name = "bcrypt"
 version = "0.17.1"