Add external link field for archives (Issue #151)

Allow users to manually set external URLs for archives from Printables,
Thingiverse, or other sources. The Globe button now opens the external
link when set, falling back to auto-detected MakerWorld URL.

- Add external_url field to PrintArchive model with migration
- Add input field to archive edit modal
- Update Globe button: external_url > makerworld_url > disabled
- Include external_url in backup/restore
- Add API test for external_url update
- Update docs (changelog, wiki, website)

Closes #151

frontend/src/api/client.ts

@@ -259,6 +259,7 @@ export interface Archive {
   extra_data: Record<string, unknown> | null;
   makerworld_url: string | null;
   designer: string | null;
+  external_url: string | null;
   is_favorite: boolean;
   tags: string | null;
   notes: string | null;
@@ -1693,6 +1694,7 @@ export const api = {
     failure_reason?: string | null;
     status?: string;
     quantity?: number;
+    external_url?: string | null;
   }) =>
     request<Archive>(`/archives/${id}`, {
       method: 'PATCH',