Switch to axum and session-rs 0.2 with a health endpoint

- Serve the session WebSocket on `/` through axum
  (`Session::from_axum`) with a 1 MiB message limit
- Add `/health` (checks the database) and a `server healthcheck`
  subcommand used by the Dockerfile HEALTHCHECK
- Shut down on SIGTERM/Ctrl+C so container stops are immediate
- Move per-connection handler registration into `register_handlers`

Co-Authored-By: Claude Opus 5.5 <[email protected]>

Cargo.toml

@@ -4,13 +4,14 @@ version = "0.1.0"
 edition = "2024"
 
 [dependencies]
+axum = { version = "0.8.9", features = ["ws"] }
 reqwest = { version = "0.13.2", features = ["json"] }
 serde = { version = "1.0.228", features = ["serde_derive"] }
 serde_json = "1.0.149"
-session-rs = "0.1.3"
+session-rs = { version = "0.2.0", default-features = false, features = ["axum"] }
 sqlx = { version = "0.8.6", features = [
     "sqlite",
     "runtime-tokio-native-tls",
     "macros",
 ] }
-tokio = "1.49.0"
+tokio = { version = "1.49.0", features = ["macros", "rt-multi-thread", "net", "signal", "sync", "time"] }