fix(db): add missing chat_messages.workflow column

User messages are persisted with a `workflow` field
(backend/src/routes/chat.ts, projectChat.ts) and it is read back when
rendering chat history (ChatView -> UserMessage) to show which workflow a
message was sent under. But the column is never created by schema.sql or any
migration, so every user-message insert fails with PostgREST PGRST204
("Could not find the 'workflow' column of 'chat_messages'") and is dropped
silently (the insert result is not checked). The assistant insert has no
workflow column, so it succeeds — the net effect on a self-hosted install is
that reloading a thread shows the assistant reply but not the user's prompt.

Add the `workflow jsonb` column to schema.sql (fresh installs) and a
migration (existing installs), mirroring the adjacent content/files jsonb
columns.

backend/migrations/20260716_01_chat_messages_workflow.sql

@@ -0,0 +1,12 @@
+-- Add chat_messages.workflow.
+--
+-- The app persists a `workflow` field on user messages
+-- (backend/src/routes/chat.ts, projectChat.ts) and reads it back when rendering
+-- chat history (frontend ChatView -> UserMessage, to show which workflow the
+-- message was sent under). The column was referenced in code but never created
+-- by schema.sql or any migration, so every user-message insert failed with
+-- PostgREST PGRST204 ("Could not find the 'workflow' column") and was dropped
+-- silently — user prompts vanished on reload while the assistant reply
+-- persisted. Mirrors the existing content/files jsonb columns.
+alter table public.chat_messages
+  add column if not exists workflow jsonb;

backend/schema.sql

@@ -524,6 +524,7 @@ create table if not exists public.chat_messages (
   role text not null,
   content jsonb,
   files jsonb,
+  workflow jsonb,
   citations jsonb,
   created_at timestamptz not null default now()
 );