Support external_ref get-or-create on POST /chat/create

From the PR description

Summary

• Adds an optional external_ref field to POST /chat/create. When provided, a repeat call with the same (user_id, external_ref) pair returns the existing chat's id instead of creating a new one.
• Lets an external integration (e.g. a Slack thread bridge) key a chat by a stable external identifier without tracking chat_id itself.
• No behavior change when external_ref is omitted.

Test plan

• New tests: get-or-create returns the same id on repeat calls with the same external_ref; a fresh external_ref creates a new chat; omitting external_ref entirely is unaffected (regression check)
• npx vitest run src/routes/__tests__/chat.test.ts - 3/3 passing
• Full backend suite run with no regressions

🤖 Generated with Claude Code