Add config.toml support for OIDC and server configuration

- Add toml crate dependency for TOML file parsing
- Create config module with Config struct to deserialize config.toml
- Support OIDC URLs (auth_url, token_url, redirect_url), credentials (client_id, client_secret)
- Make server host/port, database URL, and session secret configurable
- Create config.example.toml with all configuration options documented
- Update main.rs to load config.toml with fallback to environment variables
- Maintain backward compatibility with environment variable configuration

Cargo.toml

@@ -16,4 +16,5 @@ tokio = { version = "1.0", features = ["full"] }
 thiserror = "2.0"
 anyhow = "1.0"
 serde = { version = "1.0", features = ["derive"] }
+toml = "0.8"
 tower-http = { version = "0.6", features = ["fs", "cors"] }