Add extensible notification providers + UI/API

Introduce a provider-based notification system and wire it through the API and admin UI. Added INotificationProvider + notification service implementation and providers (apprise, discord, ntfy, pushover), plus a GET /api/admin/notifications/providers endpoint to expose provider metadata. Refactored code to use provider type strings (removed enum coupling), updated masking/encryption calls, and simplified the test notification endpoint to accept backendId or type+config and call sendToBackend directly.

UI: NotificationsTab now fetches provider metadata and renders provider cards and dynamic config forms (fields driven by provider metadata). Added config field rendering, improved backend cards, and edit/delete actions.

APIs: New providers route, updated admin notification CRUD routes to validate provider types dynamically, updated test route schema. Added download-client categories POST API to fetch categories from clients and wired postImportCategory handling in download-client routes.

Other notable changes: BookDate now fetches Claude models dynamically from Anthropic's Models API; added paginated model fetch helper. Added ALLOW_WEAK_PASSWORD flag exposure to auth providers and password change logic. Doc updates and various tests added/updated. File-organization doc clarifies EPERM fix using stream-based copy.

src/lib/services/notification/index.ts

@@ -0,0 +1,36 @@
+/**
+ * Notification Service - Public API
+ * Documentation: documentation/backend/services/notifications.md
+ */
+
+// Interface + shared types
+export type {
+  INotificationProvider,
+  NotificationEvent,
+  NotificationBackendType,
+  NotificationPayload,
+  ProviderConfigField,
+  ProviderMetadata,
+} from './INotificationProvider';
+
+// Core service
+export {
+  NotificationService,
+  getNotificationService,
+  registerProvider,
+  getProvider,
+  getRegisteredProviderTypes,
+  getAllProviderMetadata,
+} from './notification.service';
+
+// Provider types
+export type { AppriseConfig } from './providers/apprise.provider';
+export type { DiscordConfig } from './providers/discord.provider';
+export type { NtfyConfig } from './providers/ntfy.provider';
+export type { PushoverConfig } from './providers/pushover.provider';
+
+// Provider classes
+export { AppriseProvider } from './providers/apprise.provider';
+export { DiscordProvider } from './providers/discord.provider';
+export { NtfyProvider } from './providers/ntfy.provider';
+export { PushoverProvider } from './providers/pushover.provider';