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

internal/config/config.go

@@ -26,6 +26,7 @@ type Config struct {
 	Server            ServerConfig       `description:"Server configuration." yaml:"server"`
 	Auth              AuthConfig         `description:"Authentication configuration." yaml:"auth"`
 	OAuth             OAuthConfig        `description:"OAuth configuration." yaml:"oauth"`
+	OIDC              OIDCConfig         `description:"OIDC provider configuration." yaml:"oidc"`
 	UI                UIConfig           `description:"UI customization." yaml:"ui"`
 	Ldap              LdapConfig         `description:"LDAP configuration." yaml:"ldap"`
 	Experimental      ExperimentalConfig `description:"Experimental features, use with caution." yaml:"experimental"`
@@ -68,6 +69,24 @@ type LdapConfig struct {
 	SearchFilter string `description:"LDAP search filter." yaml:"searchFilter"`
 }
 
+type OIDCConfig struct {
+	Enabled          bool                        `description:"Enable OIDC provider functionality." yaml:"enabled"`
+	Issuer           string                      `description:"OIDC issuer URL (defaults to appUrl)." yaml:"issuer"`
+	AccessTokenExpiry int                        `description:"Access token expiry time in seconds." yaml:"accessTokenExpiry"`
+	IDTokenExpiry     int                        `description:"ID token expiry time in seconds." yaml:"idTokenExpiry"`
+	Clients           map[string]OIDCClientConfig `description:"OIDC client configurations." yaml:"clients"`
+}
+
+type OIDCClientConfig struct {
+	ClientSecret     string   `description:"OIDC client secret." yaml:"clientSecret"`
+	ClientSecretFile string   `description:"Path to the file containing the OIDC client secret." yaml:"clientSecretFile"`
+	ClientName       string   `description:"Client name for display purposes." yaml:"clientName"`
+	RedirectURIs     []string `description:"Allowed redirect URIs." yaml:"redirectUris"`
+	GrantTypes       []string `description:"Allowed grant types (defaults to ['authorization_code'])." yaml:"grantTypes"`
+	ResponseTypes    []string `description:"Allowed response types (defaults to ['code'])." yaml:"responseTypes"`
+	Scopes           []string `description:"Allowed scopes (defaults to ['openid', 'profile', 'email'])." yaml:"scopes"`
+}
+
 type ExperimentalConfig struct {
 	ConfigFile string `description:"Path to config file." yaml:"-"`
 }