web: support busy session metadata edits safely

Summary

The web UI currently blocks session metadata edits while a session worker is busy. We initially tried to allow state-only edits such as rename, archive / unarchive, and title generation during busy sessions, but review found that the current state.json read-modify-write paths can lose concurrent updates.

This issue now tracks the larger fix: supporting busy-session metadata edits safely, without rolling back worker-owned or web-owned state.

Current behavior

When a session is busy, web endpoints that use get_editable_session reject metadata edits with:

Session is busy. Please wait for it to complete before modifying.

That remains the safer behavior until session-state writes have proper synchronization or field-level merge semantics.

Desired behavior

Allow state-only metadata edits while a session worker is running, including:

• rename session
• archive / unarchive session
• generate session title

Delete / fork / upload may still stay blocked, because those operations have stronger conflicts with a running worker.

Risks to solve first

• PATCH /api/sessions/{session_id} currently performs an unlocked load / mutate / save of the full session state, so it can overwrite worker-owned fields saved concurrently.
• POST /api/sessions/{session_id}/generate-title also writes state after an async LLM call and needs protection against concurrent worker saves.
• Not all worker writes go through the same merge path; for example, first-turn auto-title has used its own load / save cycle.

Possible approaches

• Add a field-level state patch helper for externally mutable metadata fields.
• Add file locking or CAS around session-state writes.
• Route all worker and web state writes through one merge-aware API.
• Add regression tests covering busy rename / archive / generate-title concurrent with worker saves.

Related work

PR #2102 keeps the busy-session guard in place, but fixes a smaller title race where a late AI-generated title could overwrite a title finalized by another request.

Start by locating get_editable_session, the PATCH session endpoint, and the generate-title endpoint, then trace their state load and save paths alongside worker writes. The issue suggests field-level patches, locking or CAS, and a shared merge-aware API; done means busy rename, archive, and title generation cannot overwrite concurrent worker-owned state, with regression tests covering those races.