feat: Add initial project structure and dependencies

This commit introduces the foundational elements of the project:
- `.gitignore` file to exclude build artifacts and environment
  variables.
- `Cargo.lock` and `Cargo.toml` defining project dependencies and
  metadata.
- `diesel.toml` for Diesel CLI configuration.
- Initial migration files (`down.sql`, `up.sql`) for database schema
  setup.
- `rustfmt.toml` for code formatting.
- Basic module structure for database, routes, schema, state, and types.
- `src/main.rs` with basic Axum server setup and dotenv loading.
- `src/routes/system_router.rs` for basic API endpoint.
- `src/state.rs` for managing application state and database connection
  pool.
- Type definitions for Subsonic API responses and extensions.

src/main.rs

@@ -0,0 +1,27 @@
+mod db;
+mod routes;
+mod schema;
+mod state;
+mod types;
+use std::env;
+
+use axum::Router;
+use dotenvy::dotenv;
+
+use crate::routes::system_router::system_routers;
+use crate::state::AppState;
+
+#[tokio::main]
+async fn main() {
+    dotenv().ok();
+    let database_url = env::var("DATABASE_URL").expect("DATABASE_URL must be set");
+    let state = AppState::new(&database_url)
+        .unwrap_or_else(|_| panic!("Error creating pool for {}", database_url));
+    tracing_subscriber::fmt::init();
+    let app = Router::new()
+        .nest("/rest", system_routers())
+        .with_state(state);
+
+    let listener = tokio::net::TcpListener::bind("0.0.0.0:3311").await.unwrap();
+    let _ = axum::serve(listener, app).await;
+}