Add OIDC provider functionality with validation setup

This commit adds OpenID Connect (OIDC) provider functionality to tinyauth,
allowing it to act as an OIDC identity provider for other applications.

Features:
- OIDC discovery endpoint at /.well-known/openid-configuration
- Authorization endpoint for OAuth 2.0 authorization code flow
- Token endpoint for exchanging authorization codes for tokens
- ID token generation with JWT signing
- JWKS endpoint for public key distribution
- Support for PKCE (code challenge/verifier)
- Nonce validation for ID tokens
- Configurable OIDC clients with redirect URIs, scopes, and grant types

Validation:
- Docker Compose setup for local testing
- OIDC test client (oidc-whoami) with session management
- Nginx reverse proxy configuration
- DNS server (dnsmasq) for custom domain resolution
- Chrome launch script for easy testing

Configuration:
- OIDC configuration in config.yaml
- Example configuration in config.example.yaml
- Database migrations for OIDC client storage

validation/nginx.conf

@@ -0,0 +1,43 @@
+events {
+    worker_connections 1024;
+}
+
+http {
+    # Use Docker's built-in DNS (127.0.0.11) for service name resolution
+    # This allows nginx to resolve Docker service names like "tinyauth" and "oidc-whoami"
+    resolver 127.0.0.11 valid=10s;
+    resolver_timeout 5s;
+
+    server {
+        listen 80;
+        server_name auth.example.com;
+
+        location / {
+            # Use variable to enable dynamic resolution at request time
+            set $backend "tinyauth:3000";
+            proxy_pass http://$backend;
+            proxy_set_header Host $host;
+            proxy_set_header X-Real-IP $remote_addr;
+            proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+            proxy_set_header X-Forwarded-Proto $scheme;
+            proxy_set_header X-Forwarded-Host $host;
+        }
+    }
+
+    server {
+        listen 80;
+        server_name client.example.com;
+
+        location / {
+            # Use variable to enable dynamic resolution at request time
+            set $backend "oidc-whoami:8765";
+            proxy_pass http://$backend;
+            proxy_set_header Host $host;
+            proxy_set_header X-Real-IP $remote_addr;
+            proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+            proxy_set_header X-Forwarded-Proto $scheme;
+            proxy_set_header X-Forwarded-Host $host;
+        }
+    }
+}
+