feat(auth): add TOTP 2FA setup and login verification

- add user twofa fields and migration

- add 2FA setup/enable/disable endpoints

- enforce OTP on login when 2FA enabled

- add web login OTP field and settings UI

app/users/models.py

@@ -24,6 +24,8 @@ class User(Base):
     bio: Mapped[str | None] = mapped_column(String(500), nullable=True)
     email_verified: Mapped[bool] = mapped_column(Boolean, default=False, nullable=False, index=True)
     allow_private_messages: Mapped[bool] = mapped_column(Boolean, default=True, nullable=False, server_default="true")
+    twofa_enabled: Mapped[bool] = mapped_column(Boolean, default=False, nullable=False, server_default="false")
+    twofa_secret: Mapped[str | None] = mapped_column(String(64), nullable=True)
     created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), server_default=func.now(), nullable=False)
     updated_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True),