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/schema.rs

@@ -0,0 +1,34 @@
+// @generated automatically by Diesel CLI.
+
+diesel::table! {
+    user_music_folders (user_id, music_folder_id) {
+        user_id -> Integer,
+        music_folder_id -> Integer,
+    }
+}
+
+diesel::table! {
+    users (id) {
+        id -> Nullable<Integer>,
+        username -> Text,
+        password -> Text,
+        email -> Text,
+        ldapAuthenticated -> Bool,
+        adminRole -> Bool,
+        settingsRole -> Bool,
+        streamRole -> Bool,
+        jukeboxRole -> Bool,
+        downloadRole -> Bool,
+        uploadRole -> Bool,
+        playlistRole -> Bool,
+        coverArtRole -> Bool,
+        commentRole -> Bool,
+        podcastRole -> Bool,
+        shareRole -> Bool,
+        videoConversionRole -> Bool,
+    }
+}
+
+diesel::joinable!(user_music_folders -> users (user_id));
+
+diesel::allow_tables_to_appear_in_same_query!(user_music_folders, users,);