feat(config): OIDC provider config skeleton (ADR-0028 Phase B.1 prep) (#64)

Co-authored-by: Gabriel Radureau <arcodange@gmail.com>
Co-committed-by: Gabriel Radureau <arcodange@gmail.com>

pkg/config/config.go

@@ -109,6 +109,7 @@ type AuthConfig struct {
 	JWT                 JWTConfig       `mapstructure:"jwt"`
 	Email               EmailConfig     `mapstructure:"email"`
 	MagicLink           MagicLinkConfig `mapstructure:"magic_link"`
+	OIDC                OIDCConfig      `mapstructure:"oidc"`
 }
 
 // MagicLinkConfig holds passwordless-auth magic-link parameters (ADR-0028 Phase A).
@@ -117,6 +118,19 @@ type MagicLinkConfig struct {
 	BaseURL string        `mapstructure:"base_url"`
 }
 
+// OIDCConfig holds OpenID Connect provider configuration (ADR-0028 Phase B).
+// Multiple providers are supported via a map keyed by provider name (e.g. "arcodange-sso", "google").
+type OIDCConfig struct {
+	Providers map[string]OIDCProvider `mapstructure:"providers"`
+}
+
+// OIDCProvider describes a single OIDC provider's discovery + client config.
+type OIDCProvider struct {
+	IssuerURL    string `mapstructure:"issuer_url"`
+	ClientID     string `mapstructure:"client_id"`
+	ClientSecret string `mapstructure:"client_secret"`
+}
+
 // EmailConfig holds outgoing email transport configuration.
 // Defaults match local Mailpit (cf. ADR-0029) so dev needs no extra setup.
 type EmailConfig struct {
@@ -287,6 +301,10 @@ func LoadConfig() (*Config, error) {
 	v.SetDefault("auth.magic_link.ttl", 15*time.Minute)
 	v.SetDefault("auth.magic_link.base_url", "http://localhost:8080")
 
+	// OIDC defaults (ADR-0028 Phase B). Providers map is empty by default;
+	// configured per environment via config file or env vars.
+	v.SetDefault("auth.oidc.providers", map[string]interface{}{})
+
 	// Check for custom config file path via environment variable
 	if configFile := os.Getenv("DLC_CONFIG_FILE"); configFile != "" {
 		v.SetConfigFile(configFile)
@@ -343,6 +361,13 @@ func LoadConfig() (*Config, error) {
 	// Magic-link environment variables (ADR-0028 Phase A).
 	v.BindEnv("auth.magic_link.ttl", "DLC_AUTH_MAGIC_LINK_TTL")
 	v.BindEnv("auth.magic_link.base_url", "DLC_AUTH_MAGIC_LINK_BASE_URL")
+
+	// OIDC environment variables (ADR-0028 Phase B). One canonical "default"
+	// provider is bindable via env; additional providers must be defined in config.yaml.
+	v.BindEnv("auth.oidc.providers.default.issuer_url", "DLC_AUTH_OIDC_ISSUER_URL")
+	v.BindEnv("auth.oidc.providers.default.client_id", "DLC_AUTH_OIDC_CLIENT_ID")
+	v.BindEnv("auth.oidc.providers.default.client_secret", "DLC_AUTH_OIDC_CLIENT_SECRET")
+
 	v.BindEnv("telemetry.sampler.type", "DLC_TELEMETRY_SAMPLER_TYPE")
 	v.BindEnv("telemetry.sampler.ratio", "DLC_TELEMETRY_SAMPLER_RATIO")
 
@@ -494,6 +519,15 @@ func (c *Config) GetMagicLinkConfig() MagicLinkConfig {
 	return out
 }
 
+// GetOIDCProviders returns the configured OIDC providers, keyed by provider name.
+// Empty map (not nil) is returned when no providers are configured.
+func (c *Config) GetOIDCProviders() map[string]OIDCProvider {
+	if c.Auth.OIDC.Providers == nil {
+		return map[string]OIDCProvider{}
+	}
+	return c.Auth.OIDC.Providers
+}
+
 // GetJWTTTL returns the JWT TTL
 func (c *Config) GetJWTTTL() time.Duration {
 	if c.Auth.JWT.TTL == 0 {