Build post chat and attachment workflows

backend/migrations/007_chat_message_features.sql

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+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);