feat: add thread deletion (MCP tool + REST API + web UI)

[bertheto]

Summary

Adds the ability to permanently delete a thread and all its messages,
consistently across all interaction surfaces.

Currently, threads can only be archived or closed (reversible). This PR
adds irreversible deletion for clean-up use cases (test threads, accidental
threads, stale content).

Changes

src/db/crud.py — thread_delete()

• Deletes messages before thread (FK constraint)
• Single transaction with rollback on error
• Returns audit dict {thread_id, topic, message_count} or None if not found
• Emits thread.deleted SSE event

src/mcp_server.py — thread_delete tool

• Requires confirm: true parameter (safeguard against accidental deletion)
• Description explicitly contrasts with thread_archive for reversible removal

src/tools/dispatch.py — handle_thread_delete()

• Returns structured error if confirm != true
• Returns audit info on success

src/main.py — DELETE /api/threads/{thread_id}

• Returns {ok: true, deleted: {thread_id, topic, message_count}}
• Returns 404 if thread not found

src/static/index.html — Web UI

• Delete button in thread context menu (red, separated by <hr>)
• Native <dialog> confirmation showing topic + message count
• SSE handler for thread.deleted clears active message pane

Design Notes

• confirm=true on MCP tool: Standard MCP pattern for destructive actions,
  prevents accidental deletion by AI agents that auto-call tools.
• Native <dialog>: No external deps, accessible, keyboard-navigable.
• No cascade FK: SQLite FK cascade is opt-in; explicit deletion order
  (messages → thread) is safer and more portable.

Test

End-to-end (HTTP):

# Create thread, post message, delete
```t = Invoke-RestMethod http://localhost:39765/api/threads -Method POST -ContentType application/json -Body '{topic:test,initiator:me}'
Invoke-RestMethod http://localhost:39765/api/threads/```(```t.id)/messages -Method POST -ContentType application/json -Body '{author:me,content:hi}'
Invoke-RestMethod http://localhost:39765/api/threads/```(```t.id) -Method DELETE
# → {ok: True, deleted: {topic: test, message_count: 1}}