feat(invite): add invite code registration system

app/models.py

@@ -1,6 +1,6 @@
 from datetime import datetime, timezone
 
-from sqlalchemy import Column, DateTime, ForeignKey, Integer, String
+from sqlalchemy import Boolean, Column, DateTime, ForeignKey, Integer, String
 from sqlalchemy.orm import relationship, validates
 
 from app.database import Base
@@ -16,11 +16,27 @@ class User(Base):
     id = Column(Integer, primary_key=True, index=True)
     username = Column(String(64), unique=True, nullable=False, index=True)
     password_hash = Column(String(128), nullable=False)
+    is_admin = Column(Boolean, nullable=False, default=False)
     showcase_mode = Column(String(16), nullable=False, default="profile")
     created_at = Column(DateTime, default=_utcnow)
 
     profiles = relationship("Profile", back_populates="user", cascade="all, delete-orphan")
     sessions = relationship("UserSession", back_populates="user", cascade="all, delete-orphan")
+    invite_codes = relationship("InviteCode", back_populates="creator", cascade="all, delete-orphan")
+
+
+class InviteCode(Base):
+    __tablename__ = "invite_codes"
+
+    id = Column(Integer, primary_key=True, index=True)
+    code = Column(String(8), unique=True, nullable=False, index=True)
+    max_uses = Column(Integer, nullable=True)  # None = unlimited
+    expires_at = Column(DateTime, nullable=True)  # None = no expiration
+    use_count = Column(Integer, nullable=False, default=0)
+    created_by = Column(Integer, ForeignKey("users.id", ondelete="SET NULL"), nullable=True)
+    created_at = Column(DateTime, default=_utcnow)
+
+    creator = relationship("User", back_populates="invite_codes")
 
 
 class UserSession(Base):