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/messages/models.py

@@ -49,7 +49,7 @@ class Message(Base):
         nullable=False,
     )
 
-    chat: Mapped["Chat"] = relationship(back_populates="messages")
+    chat: Mapped["Chat"] = relationship(back_populates="messages", foreign_keys=[chat_id])
     sender: Mapped["User"] = relationship(back_populates="sent_messages")
     attachments: Mapped[list["Attachment"]] = relationship(back_populates="message", cascade="all, delete-orphan")