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/tests/test_admin.py

@@ -15,7 +15,7 @@ class TestAdminEndpoints:
     @pytest.mark.asyncio
     async def test_list_users_admin(self, client: AsyncClient, admin_headers: dict):
         """Test listing users as admin."""
-        response = await client.get("/admin/users", headers=admin_headers)
+        response = await client.get("/users/users", headers=admin_headers)
         print(f"\nDEBUG: status={response.status_code}, body={response.text}")
         if response.status_code != 200:
             print(f"DEBUG: response.json()={response.json()}")
@@ -26,14 +26,14 @@ class TestAdminEndpoints:
     @pytest.mark.asyncio
     async def test_list_users_non_admin(self, client: AsyncClient, auth_headers: dict):
         """Test listing users as non-admin."""
-        response = await client.get("/admin/users", headers=auth_headers)
+        response = await client.get("/users/users", headers=auth_headers)
         assert response.status_code == 403
     
     @pytest.mark.asyncio
     async def test_create_ban(self, client: AsyncClient, admin_headers: dict, test_user: User):
         """Test creating a ban."""
         response = await client.post(
-            "/admin/bans",
+            "/users/bans",
             json={
                 "user_id": test_user.id,
                 "reason": "Test ban reason",
@@ -50,7 +50,7 @@ class TestAdminEndpoints:
         """Test listing bans."""
         # Create a ban first
         await client.post(
-            "/admin/bans",
+            "/users/bans",
             json={
                 "user_id": test_user.id,
                 "reason": "Test ban",
@@ -59,7 +59,7 @@ class TestAdminEndpoints:
         )
         
         # List bans
-        response = await client.get("/admin/bans", headers=admin_headers)
+        response = await client.get("/users/bans", headers=admin_headers)
         assert response.status_code == 200
         data = response.json()
         assert isinstance(data, list)
@@ -70,7 +70,7 @@ class TestAdminEndpoints:
         """Test unbanning a user."""
         # Create a ban first
         ban_response = await client.post(
-            "/admin/bans",
+            "/users/bans",
             json={
                 "user_id": test_user.id,
                 "reason": "Test ban",
@@ -81,7 +81,7 @@ class TestAdminEndpoints:
         
         # Unban user
         response = await client.post(
-            f"/admin/bans/{ban_id}/unban",
+            f"/users/bans/{ban_id}/unban",
             headers=admin_headers,
         )
         assert response.status_code == 200