Store: add repositories in a background job and show the TOFU fingerprint

Adding a repository downloads and verifies its whole index, so it now runs as a
job (runJob in the UI) and reports 'Trusted on first use: <fingerprint>' when
no pin was given. fdroidrepos:// links pass the server check, as documented.
Jobs report SourceError messages without a 'SourceError:' prefix.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

ui/apk_sources/search.py

@@ -98,10 +98,16 @@ def set_enabled(source_id, enabled):
     return {'message': source['name'] + (' enabled' if enabled else ' disabled')}
 
 
+def repo_module():
+    module = next((m for m in modules()[0] if m.KIND == 'fdroid'), None)
+    if module is None or not hasattr(module, 'add_repo'):
+        raise SourceError('User repositories are not available in this build')
+    return module
+
+
 def manage_repo(action, **kwargs):
-    mods, _ = modules()
-    module = next((m for m in mods if m.KIND == 'fdroid'), None)
-    if module is None or not hasattr(module, action):
+    module = repo_module()
+    if not hasattr(module, action):
         raise SourceError('User repositories are not available in this build')
     return getattr(module, action)(**kwargs)