fix: Add explicit foreign_keys to User.containers relationship

SQLAlchemy konnte nicht bestimmen welcher FK gemeint ist, da UserContainer
zwei FKs zu User hat (user_id und blocked_by).

Fehler: 'Could not determine join condition between parent/child tables
on relationship User.containers - there are multiple foreign key paths'

Lösung: foreign_keys=[user_id] explizit angeben.

models.py

@@ -149,7 +149,7 @@ class UserContainer(db.Model):
     blocked_by = db.Column(db.Integer, db.ForeignKey('user.id', ondelete='SET NULL'), nullable=True)
 
     # Relationships
-    user = db.relationship('User', back_populates='containers')
+    user = db.relationship('User', foreign_keys=[user_id], back_populates='containers')
     blocker = db.relationship('User', foreign_keys=[blocked_by])
 
     # Unique: Ein User kann nur einen Container pro Typ haben