feat(auth): add OpenID Connect (OIDC) authentication support

Implements OIDC Authorization Code Flow for external identity providers
(Authentik, Keycloak, etc.) with JIT user provisioning.

New features:
- OidcService with OpenID Discovery, JWKS caching, RS256 ID token validation
- Authorization Code Flow: /api/auth/oidc/authorize -> IdP -> /api/auth/oidc/callback
- JIT user provisioning from OIDC claims (sub, email, name, groups)
- OIDC group-to-role mapping (admin_groups config)
- Provider info endpoint: GET /api/auth/oidc/providers
- Option to disable password login entirely (OXICLOUD_OIDC_DISABLE_PASSWORD_LOGIN)
- Auto-provision toggle (OXICLOUD_OIDC_AUTO_PROVISION)
- Email collision detection (security: prevents account takeover)

Configuration (env vars):
- OXICLOUD_OIDC_ENABLED, OXICLOUD_OIDC_ISSUER_URL
- OXICLOUD_OIDC_CLIENT_ID, OXICLOUD_OIDC_CLIENT_SECRET
- OXICLOUD_OIDC_REDIRECT_URI, OXICLOUD_OIDC_SCOPES
- OXICLOUD_OIDC_FRONTEND_URL, OXICLOUD_OIDC_PROVIDER_NAME
- OXICLOUD_OIDC_AUTO_PROVISION, OXICLOUD_OIDC_ADMIN_GROUPS
- OXICLOUD_OIDC_DISABLE_PASSWORD_LOGIN

DB migration:
- ALTER TABLE auth.users ADD oidc_provider, oidc_subject columns
- UNIQUE index on (oidc_provider, oidc_subject)

Files changed: 14 files, ~1400 lines added
Dependencies: reqwest 0.12 (rustls-tls-webpki-roots), base64 0.22

src/application/dtos/user_dto.rs

@@ -74,4 +74,41 @@ pub struct CurrentUser {
     pub username: String,
     pub email: String,
     pub role: String,
+}
+
+// ============================================================================
+// OIDC DTOs
+// ============================================================================
+
+/// Response with the OIDC authorization URL for client redirect
+#[derive(Debug, Serialize, Deserialize)]
+pub struct OidcAuthorizeResponseDto {
+    pub authorize_url: String,
+    pub state: String,
+}
+
+/// Query parameters received on the OIDC callback
+#[derive(Debug, Serialize, Deserialize)]
+pub struct OidcCallbackQueryDto {
+    pub code: String,
+    pub state: String,
+}
+
+/// Information about available OIDC providers
+#[derive(Debug, Serialize, Deserialize)]
+pub struct OidcProviderInfoDto {
+    pub enabled: bool,
+    pub provider_name: String,
+    pub authorize_endpoint: String,
+    pub password_login_enabled: bool,
+}
+
+/// Claims extracted from the validated OIDC ID token
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct OidcUserInfoDto {
+    pub sub: String,
+    pub preferred_username: Option<String>,
+    pub email: Option<String>,
+    pub name: Option<String>,
+    pub groups: Vec<String>,
 }