feat(core): clear saved chat and add message deletion scopes

backend:

- add message_hidden table for per-user message hiding

- support DELETE /messages/{id}?for_all=true|false

- implement delete-for-me vs delete-for-all logic by chat type/permissions

- add POST /chats/{chat_id}/clear and route saved chat deletion to clear

web:

- saved messages action changed from delete to clear

- message context menu now supports delete modal: for me / for everyone

- add local store helpers removeMessage/clearChatMessages

- include realtime stability improvements and app error boundary

app/messages/models.py

@@ -83,3 +83,13 @@ class MessageReceipt(Base):
         onupdate=func.now(),
         nullable=False,
     )
+
+
+class MessageHidden(Base):
+    __tablename__ = "message_hidden"
+    __table_args__ = (UniqueConstraint("message_id", "user_id", name="uq_message_hidden_message_user"),)
+
+    id: Mapped[int] = mapped_column(primary_key=True, index=True)
+    message_id: Mapped[int] = mapped_column(ForeignKey("messages.id", ondelete="CASCADE"), nullable=False, index=True)
+    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)