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

backend/app/core/database.py

@@ -689,6 +689,12 @@ async def run_migrations(conn):
     except Exception:
         pass
 
+    # Migration: Add external_url column to print_archives for user-defined links (Printables, etc.)
+    try:
+        await conn.execute(text("ALTER TABLE print_archives ADD COLUMN external_url VARCHAR(500)"))
+    except Exception:
+        pass
+
 
 async def seed_notification_templates():
     """Seed default notification templates if they don't exist."""