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.

migrations/2026-02-09-205759-0000_init/down.sql

@@ -0,0 +1,4 @@
+-- This file should undo anything in `up.sql`
+-- down.sql (Diesel)
+DROP TABLE IF EXISTS user_music_folders;
+DROP TABLE IF EXISTS users;