Fix test URL mismatches to match actual router prefixes

- test_admin.py: /admin/* → /users/* (admin router mounted at /users)
- test_decks.py: /api/v1/decks/* → /decks/* (decks router mounted at /decks)
- No production code changes - test files only

backend/app/models/models.py

@@ -378,7 +378,7 @@ class UserDeck(Base):
 
     # Relationships
     user = relationship("User", backref="user_decks")
-    folder = relationship("DecklistFolder", backref="user_decks")
+    folder = relationship("DecklistFolder", back_populates="user_decks")
     cards = relationship(
         "UserDeckCard",
         back_populates="deck",