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

src/lib.rs

@@ -1,5 +1,6 @@
 pub mod handlers;
 pub mod models;
+pub mod config;
 
 use axum::{
     Router,