Revert "Build post chat and attachment workflows"

This reverts commit 0b920da187.

backend/migrations/007_chat_message_features.sql

@@ -1,19 +0,0 @@
-ALTER TABLE calendar_item_chat_messages
-ADD COLUMN IF NOT EXISTS edited_at TIMESTAMPTZ,
-ADD COLUMN IF NOT EXISTS deleted_at TIMESTAMPTZ;
-
-CREATE TABLE IF NOT EXISTS calendar_item_chat_attachments (
-  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
-  chat_message_id UUID NOT NULL REFERENCES calendar_item_chat_messages(id) ON DELETE CASCADE,
-  original_filename TEXT NOT NULL,
-  stored_filename TEXT NOT NULL,
-  mime_type TEXT NOT NULL,
-  size_bytes BIGINT NOT NULL CHECK (size_bytes >= 0),
-  storage_driver TEXT NOT NULL,
-  storage_path TEXT NOT NULL,
-  created_by UUID REFERENCES users(id) ON DELETE SET NULL,
-  created_at TIMESTAMPTZ NOT NULL DEFAULT now()
-);
-
-CREATE INDEX IF NOT EXISTS idx_calendar_item_chat_attachments_message
-  ON calendar_item_chat_attachments (chat_message_id, created_at);