feat: remove username uniqueness constraint for community edition

- Remove unique=True from username column in User model
- Remove username duplicate check in create_user and create_superuser
- Add migration to drop unique index on username, keep email unique

api/app/models/user_model.py

@@ -9,7 +9,7 @@ class User(Base):
     __tablename__ = "users"
 
     id = Column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4, index=True)
-    username = Column(String, unique=True, index=True, nullable=False)
+    username = Column(String, index=True, nullable=False)  # 社区版：用户名不唯一，仅邮箱唯一
     email = Column(String, unique=True, index=True, nullable=False)
     hashed_password = Column(String, nullable=False)
     is_active = Column(Boolean, default=True, nullable=False)