refactor(models): unify table names to singular form for consistency across models

app/models/user.py

@@ -20,7 +20,7 @@ if TYPE_CHECKING:
 class User(db.Model):
     """User model for storing user information."""
 
-    __tablename__ = "users"
+    __tablename__ = "user"
 
     id: Mapped[int] = mapped_column(primary_key=True)
 
@@ -48,7 +48,7 @@ class User(db.Model):
     # Plan relationship
     plan_id: Mapped[int] = mapped_column(
         Integer,
-        ForeignKey("plans.id"),
+        ForeignKey("plan.id"),
         nullable=False,
     )