feat(privacy): user blocklist with private chat enforcement

- add blocked_users table and migration
- add users API: block, unblock, list blocked users
- prevent private chat creation and private messaging when block relation exists
- add block/unblock action in private chat info panel

app/users/models.py

@@ -1,7 +1,7 @@
 from datetime import datetime
 from typing import TYPE_CHECKING
 
-from sqlalchemy import Boolean, DateTime, String, func
+from sqlalchemy import Boolean, DateTime, ForeignKey, String, UniqueConstraint, func
 from sqlalchemy.orm import Mapped, mapped_column, relationship
 
 from app.database.base import Base
@@ -42,3 +42,13 @@ class User(Base):
         back_populates="user",
         cascade="all, delete-orphan",
     )
+
+
+class BlockedUser(Base):
+    __tablename__ = "blocked_users"
+    __table_args__ = (UniqueConstraint("user_id", "blocked_user_id", name="uq_blocked_users_user_target"),)
+
+    id: Mapped[int] = mapped_column(primary_key=True, index=True)
+    user_id: Mapped[int] = mapped_column(ForeignKey("users.id", ondelete="CASCADE"), nullable=False, index=True)
+    blocked_user_id: Mapped[int] = mapped_column(ForeignKey("users.id", ondelete="CASCADE"), nullable=False, index=True)
+    created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), server_default=func.now(), nullable=False)