fix(orm): disambiguate Chat<->Message relationships

- set explicit foreign_keys for Chat.messages and Message.chat

- resolve mapper initialization error after pinned_message_id fk

app/chats/models.py

@@ -34,7 +34,11 @@ class Chat(Base):
     created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), server_default=func.now(), nullable=False)
 
     members: Mapped[list["ChatMember"]] = relationship(back_populates="chat", cascade="all, delete-orphan")
-    messages: Mapped[list["Message"]] = relationship(back_populates="chat", cascade="all, delete-orphan")
+    messages: Mapped[list["Message"]] = relationship(
+        back_populates="chat",
+        cascade="all, delete-orphan",
+        foreign_keys="Message.chat_id",
+    )
 
 
 class ChatMember(Base):