Rewire issue tracker integration from issue-facade to issue-core

issue-facade was renamed to issue-core. Update the dependency
declaration and the three Python imports it touched. Model field
names (issue_facade_backend, issue_facade_id) and the Django setting
ISSUE_FACADE_LOCAL_DB stay as-is — they are persisted/semantic
identifiers, not part of the package wiring.

All 20 aufgaben tests pass after the rewire.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

vergabe_teilnahme/apps/aufgaben/issue_backends.py

@@ -5,7 +5,7 @@ from pathlib import Path
 @contextmanager
 def local_backend():
     from django.conf import settings
-    from issue_tracker.backends.local import LocalSQLiteBackend
+    from issue_core.backends.local import LocalSQLiteBackend
 
     db_path = str(getattr(settings, 'ISSUE_FACADE_LOCAL_DB', '.issue-facade/issues.db'))
     Path(db_path).parent.mkdir(parents=True, exist_ok=True)
@@ -22,7 +22,7 @@ def local_backend():
 def remote_backend():
     """Yields GiteaBackend wenn konfiguriert, sonst None."""
     from django.conf import settings
-    from issue_tracker.backends.gitea import GiteaBackend
+    from issue_core.backends.gitea import GiteaBackend
 
     cfg = getattr(settings, 'ISSUE_FACADE_GITEA', None)
     if not cfg:

vergabe_teilnahme/apps/aufgaben/issue_facade.py

@@ -1,6 +1,6 @@
 from datetime import datetime, timezone
 
-from issue_tracker.core.models import Issue, IssueState, Label, Priority
+from issue_core.core.models import Issue, IssueState, Label, Priority
 
 from .issue_backends import gitea_configured, local_backend, remote_backend