Isolate proactive messages by chat thread

[blackbartblues]

What changed

• binds proactive chat notifications to the thread that was active when they were created
• keeps retry delivery bound to the original thread
• preserves empty user-created conversations in the thread list
• prevents live notifications for another thread from appearing in the open transcript
• migrates legacy proactive messages to their original day thread

Root cause

Profile-wide chat_log rows were merged into whichever thread was currently active. In addition, a newly created empty thread was omitted from /api/chat/threads until its first Hermes message, so a refresh could select an older conversation again.

Impact

/new now remains a genuinely separate conversation after refresh, and old system alerts no longer reappear above every new user message.

Validation

• backend: 375 passed
• app: 218 passed
• ESLint passed
• TypeScript and Svelte checks passed
• production app build passed