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

static/index.html

@@ -23,7 +23,7 @@
 
     <!-- Splash screens for iOS -->
     <link rel="apple-touch-startup-image" href="/img/android-chrome-512x512.png" />
-    <script type="module" crossorigin src="/assets/index-Du4KZbJd.js"></script>
+    <script type="module" crossorigin src="/assets/index-BQzscOA-.js"></script>
     <link rel="stylesheet" crossorigin href="/assets/index-BKSrBx0A.css">
   </head>
   <body>