fix: pgAdapter maybeSingle, jsonb serialization for chat messages

Three bugs in local/Docker mode:

1. PgAdapter.maybeSingle() returned the full rows array instead of a
   single row because _run() only checked _singleMode, not
   _maybeSingleMode. All routes using maybeSingle() got an array back
   and treated it as "not found" → "Chat not found" on every POST /chat.

2. JavaScript arrays passed to pg as query parameters are formatted as
   PostgreSQL array literals {a,b} which is invalid for jsonb columns.
   Fixed in PgAdapter insert/update to JSON.stringify arrays before
   binding.

3. Plain strings inserted into jsonb columns (chat_messages.content for
   user messages) are not valid JSON, causing the INSERT to fail
   silently. Fixed by JSON.stringify-ing string content in chat.ts,
   projectChat.ts and tabular.ts before the INSERT.

Also added missing `workflow jsonb` column to chat_messages in both
init.sql and schema.sql - the backend was already inserting this field.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>