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/INotificationProvider.ts

@@ -0,0 +1,60 @@
+/**
+ * Notification Provider Interface
+ * Documentation: documentation/backend/services/notifications.md
+ */
+
+// Event types
+export type NotificationEvent =
+  | 'request_pending_approval'
+  | 'request_approved'
+  | 'request_available'
+  | 'request_error';
+
+// Backend type — string-based, registry is the runtime source of truth
+export type NotificationBackendType = string;
+
+// Notification payload
+export interface NotificationPayload {
+  event: NotificationEvent;
+  requestId: string;
+  title: string;
+  author: string;
+  userName: string;
+  message?: string; // For error events
+  timestamp: Date;
+}
+
+// Provider config field definition for dynamic UI rendering
+export interface ProviderConfigField {
+  name: string;
+  label: string;
+  type: 'text' | 'password' | 'select' | 'number';
+  required: boolean;
+  placeholder?: string;
+  defaultValue?: string | number;
+  options?: { label: string; value: string | number }[];
+}
+
+// Provider metadata for self-describing providers
+export interface ProviderMetadata {
+  type: string;
+  displayName: string;
+  description: string;
+  iconLabel: string;
+  iconColor: string;
+  configFields: ProviderConfigField[];
+}
+
+export interface INotificationProvider {
+  /** Provider identifier */
+  type: string;
+
+  /** Config field names that need encryption/masking */
+  sensitiveFields: string[];
+
+  /** Self-describing metadata for UI and validation */
+  metadata: ProviderMetadata;
+
+  /** Send notification with already-decrypted config */
+  send(config: Record<string, any>, payload: NotificationPayload): Promise<void>;
+}