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/messages/models.py

@@ -29,6 +29,16 @@ class Message(Base):
     id: Mapped[int] = mapped_column(primary_key=True, index=True)
     chat_id: Mapped[int] = mapped_column(ForeignKey("chats.id", ondelete="CASCADE"), nullable=False, index=True)
     sender_id: Mapped[int] = mapped_column(ForeignKey("users.id", ondelete="CASCADE"), nullable=False, index=True)
+    reply_to_message_id: Mapped[int | None] = mapped_column(
+        ForeignKey("messages.id", ondelete="SET NULL"),
+        nullable=True,
+        index=True,
+    )
+    forwarded_from_message_id: Mapped[int | None] = mapped_column(
+        ForeignKey("messages.id", ondelete="SET NULL"),
+        nullable=True,
+        index=True,
+    )
     type: Mapped[MessageType] = mapped_column(SAEnum(MessageType), nullable=False, default=MessageType.TEXT)
     text: Mapped[str | None] = mapped_column(Text, nullable=True)
     created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), server_default=func.now(), nullable=False)