feat: add reply/forward/pin message flow across backend and web

- add reply_to/forwarded_from message fields and chat pinned_message field

- add forward and pin APIs plus reply support in message create

- wire web actions: Reply, Fwd, Pin and reply composer state

- fix spam policy bug: allow repeated identical messages, keep rate limiting

app/chats/models.py

@@ -30,6 +30,7 @@ class Chat(Base):
     id: Mapped[int] = mapped_column(primary_key=True, index=True)
     type: Mapped[ChatType] = mapped_column(SAEnum(ChatType), nullable=False, index=True)
     title: Mapped[str | None] = mapped_column(String(255), nullable=True)
+    pinned_message_id: Mapped[int | None] = mapped_column(ForeignKey("messages.id", ondelete="SET NULL"), nullable=True, index=True)
     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")