Add Authentik OAuth/OIDC SSO, email change, and setup docs

Phase 18 changes:

OAuth/OIDC SSO (Authorization Code + PKCE):
- alembic/versions/0015_add_oauth_fields.py: add oauth_sub UNIQUE column,
  drop NOT NULL on password_hash
- src/fabledassistant/services/oauth.py: OIDC discovery (cached), build_auth_url,
  exchange_code, get_userinfo, find_or_create_oauth_user (sub→email auto-link→create)
- src/fabledassistant/routes/auth.py: GET /api/auth/oauth/login and
  GET /api/auth/oauth/callback; LOCAL_AUTH_ENABLED guards on login/register;
  /api/auth/status now returns oauth_enabled + local_auth_enabled
- src/fabledassistant/config.py: OIDC_ISSUER, OIDC_CLIENT_ID, OIDC_CLIENT_SECRET,
  OIDC_SCOPES, LOCAL_AUTH_ENABLED, oidc_enabled() classmethod
- src/fabledassistant/models/user.py: password_hash nullable, oauth_sub field,
  has_password bool in to_dict()
- src/fabledassistant/services/auth.py: create_user accepts password=None +
  oauth_sub kwarg; authenticate returns None for OAuth-only users;
  add get_user_by_oauth_sub, link_oauth_sub, update_user_email
- frontend: AuthStatus + User types updated; auth store exposes oauthEnabled +
  localAuthEnabled; LoginView shows SSO button / hides password form accordingly

Email change:
- PUT /api/auth/email: requires password confirmation for local-auth users,
  skips check for OAuth-only users; enforces email uniqueness
- SettingsView.vue: new Email Address section pre-filled with current email,
  updates authStore.user in-place on success

Docs:
- docs/oauth-setup.md: step-by-step Authentik provider setup, example
  docker-compose env vars, account linking explanation, per-provider issuer URL table

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/stores/auth.ts

@@ -8,6 +8,8 @@ export const useAuthStore = defineStore("auth", () => {
   const loading = ref(true);
   const hasUsers = ref(true);
   const registrationOpen = ref(false);
+  const oauthEnabled = ref(false);
+  const localAuthEnabled = ref(true);
 
   const isAuthenticated = computed(() => user.value !== null);
   const isAdmin = computed(() => user.value?.role === "admin");
@@ -28,9 +30,13 @@ export const useAuthStore = defineStore("auth", () => {
       const data = await apiGet<AuthStatus>("/api/auth/status");
       hasUsers.value = data.has_users;
       registrationOpen.value = data.registration_open;
+      oauthEnabled.value = data.oauth_enabled ?? false;
+      localAuthEnabled.value = data.local_auth_enabled ?? true;
     } catch {
       hasUsers.value = true;
       registrationOpen.value = false;
+      oauthEnabled.value = false;
+      localAuthEnabled.value = true;
     }
   }
 
@@ -56,6 +62,8 @@ export const useAuthStore = defineStore("auth", () => {
     loading,
     hasUsers,
     registrationOpen,
+    oauthEnabled,
+    localAuthEnabled,
     isAuthenticated,
     isAdmin,
     checkAuth,