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.

diesel.toml

@@ -0,0 +1,9 @@
+# For documentation on how to configure this file,
+# see https://diesel.rs/guides/configuring-diesel-cli
+
+[print_schema]
+file = "src/schema.rs"
+custom_type_derives = ["diesel::query_builder::QueryId", "Clone"]
+
+[migrations_directory]
+dir = "migrations"