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/utils/copy-file.ts

@@ -0,0 +1,22 @@
+/**
+ * Component: Stream-based File Copy Utility
+ * Documentation: documentation/phase3/file-organization.md
+ *
+ * Uses read()/write() syscalls via Node.js streams instead of fs.copyFile(),
+ * which relies on copy_file_range() — a syscall that fails with EPERM on
+ * certain filesystem configurations (e.g. cross-export NFS4 mounts).
+ */
+
+import { createReadStream, createWriteStream } from 'fs';
+import { pipeline } from 'stream/promises';
+
+/**
+ * Copy a file using streams.
+ *
+ * Equivalent to `fs.copyFile()` but uses standard read/write syscalls
+ * instead of `copy_file_range()`, ensuring compatibility with NFS, FUSE,
+ * and other network/virtual filesystems.
+ */
+export async function copyFile(source: string, destination: string): Promise<void> {
+  await pipeline(createReadStream(source), createWriteStream(destination));
+}