feat: Add pause/resume functionality for runner sessions (instead of session termination on break)

[masa10-f]

Summary

Currently, when a user takes a break during a runner session, the session is completely terminated. This requires the user to go through the full checkout flow (KPT input, etc.) and start a new session after the break.

A more natural workflow would be to simply pause the timer during breaks without ending the session.

Current Behavior

Work Start → Break Button → Checkout Dialog → Session Ends → Must start new session

• Break triggers checkout_session() with decision='break'
• Session's ended_at is set, marking session as complete
• User must create a new session to continue working

Relevant code:

• apps/api/src/humancompiler_api/services.py:1161-1290 - checkout_session method
• apps/web/src/components/runner/break-dialog.tsx - Break dialog UI
• apps/web/src/components/runner/runner-page.tsx:221-233 - Break handler

Proposed Behavior

Work Start → Break Button → Timer Pauses → Resume Button → Same session continues

• Break should pause the timer without ending the session
• User can resume the same session after the break
• Paused time should be excluded from actual_minutes calculation
• Full checkout flow only when actually finishing work

Implementation Plan

Backend Changes

1. Add fields to WorkSession model (models.py):

   paused_at: datetime | None = SQLField(default=None)
   total_paused_minutes: int = SQLField(default=0)

2. Add new API endpoints (work_sessions.py):

   • POST /work-sessions/pause - Pause current session
   • POST /work-sessions/resume - Resume paused session

3. Add service methods (services.py):

   • pause_session() - Set paused_at timestamp
   • resume_session() - Calculate paused duration, add to total_paused_minutes, clear paused_at

4. Update time calculation:

   • Modify checkout_session() to exclude total_paused_minutes from actual_minutes

Frontend Changes

1. Update use-runner.ts hook:

   • Add isPaused state
   • Add pause() and resume() functions

2. Update runner-page.tsx:

   • Show pause/resume button instead of break
   • Display paused state UI (dimmed timer, "Paused" indicator)

3. Update or replace break-dialog.tsx:

   • Simple confirmation without KPT input for pause
   • Or remove dialog entirely for instant pause

Database Migration

• Add paused_at and total_paused_minutes columns to work_sessions table

Additional Considerations

• Should there be a maximum pause duration before auto-checkout?
• Should notification system ([Sub-Issue] 通知/未応答エスカレーション: PC/スマホでチェックアウトを"見逃さない" #228) behave differently during pause?
• Keep existing "break" decision for backward compatibility or deprecate?

Related

• Issue [Sub-Issue] 通知/未応答エスカレーション: PC/スマホでチェックアウトを"見逃さない" #228 - Notification and escalation system