fix(db): add missing chat_messages.workflow column (user prompts dropped on reload)

From the PR description

Problem

User reloading a thread shows the assistant reply but not the user's own prompt - the prompt only ever existed in the browser's in-memory state during the live turn.

User messages are inserted with a workflow field:

• backend/src/routes/chat.ts - the role: "user" insert
• backend/src/routes/projectChat.ts - same

and it is read back when rendering chat history (ChatView → UserMessage) to show which workflow a message was sent under.

But chat_messages.workflow is never created - it is absent from backend/schema.sql and no migration adds it. On a fresh self-hosted install, every user-message insert fails with:

PostgREST PGRST204: Could not find the 'workflow' column of 'chat_messages' in the schema cache

The insert result isn't checked, so the failure is silent. The assistant-message insert has no workflow column, so it succeeds. Net effect: reloading a thread shows the assistant reply but not the user's own prompt - the prompt only ever existed in the browser's in-memory state during the live turn.

Reproduction

On a DB built from schema.sql:

insert into chat_messages (chat_id, role, content, files, workflow)
values ('<existing-chat-id>', 'user', '"hi"'::jsonb, null, null);
-- ERROR: PGRST204 - column "workflow" does not exist

The same insert without workflow succeeds. Querying chat_messages afterwards shows only assistant rows.

Fix

Add the workflow jsonb column, mirroring the adjacent content/files jsonb columns:

• schema.sql - for fresh installs
• migrations/20260716_01_chat_messages_workflow.sql - idempotent add column if not exists, for existing installs

After applying, user-message inserts succeed and prompts persist across reloads.

My Setup

Mike and all components were installed on a single VM using: Docker 29.6, Node 20, nginx, MinIO (S3 storage) and Supabase.

🤖 Generated with Claude Code. Reviewed by a human, me. :)