Features/jaydeep/check ins improvement

[jaydeeprusia]

What changed?

This updates the weekly check-in flow so each diagnosis has its own linked check-in, the active diagnosis/check-in pair stays editable until the next re-diagnosis, and check-in history is displayed as a collapsible timeline. The check-in UI now shows clear complete/incomplete action states and uses the checkins collection as the source of truth for history.

Why?

This was requested to make the weekly flow match the product model: idea -> diagnosis -> action set -> check-in -> re-diagnose -> next diagnosis. It also removes redundant check-in storage and makes the history easier to scan without losing the underlying diagnosis/action context.

Type of change

• Feature
• Bug fix
• Refactor with no behavior change
• Docs or chore
• Cherry-pick from local_setup

Verification

• Backend tests or relevant API checks
• Frontend build or relevant browser check
• Screenshots for meaningful UI changes

Notes:

• Focused frontend test passed: yarn test --watchAll=false IdeaDetail.checkin.test.jsx
• Backend validation passed for touched files with no errors
• Focused backend check-in test ran in this environment but the integration case was skipped here

Safety checklist

• No .env file is committed.
• No real credentials, tokens, AWS keys, OAuth secrets, or JWT secrets are committed.
• Provider abstractions in backend/auth_provider.py and backend/llm_provider.py remain intact.
• backend/.env.example keeps open-source-safe placeholder values.
• I read BRANCHING.md if this touches auth, LLM routing, env files, dependencies, or CI.

Rollback plan

Revert the check-in flow changes in backend/server.py, frontend/src/pages/IdeaDetail.jsx, and frontend/src/lib/api.js to restore the previous diagnosis/check-in behavior.

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