fix(sync): return not_started for unsynced episodes

[shark0F0497]

Pull Request Checklist

• Code follows the style guidelines
• Tests pass locally
• Code is formatted
• Documentation updated if needed
• Commit messages follow conventional commits
• PR description is complete and clear

---

Summary

This PR changes the episode cloud sync status endpoint so an existing episode with no sync log returns a normal not_started status instead of a 404.

---

Motivation

• Opening an episode detail page currently calls GET /api/v1/sync/episodes/:id/status.
• Episodes that have never been synced have no sync_logs row, which previously returned 404 no sync record found for this episode.
• That 404 is a normal business state, not an error; it creates noisy browser Network/console output and forces frontend exception handling.

---

Changes

Modified Files

• internal/api/handlers/sync.go - Return a virtual SyncJobResponse with status: "not_started" and id: 0 when the episode exists but has no sync log; preserve 404 for missing or soft-deleted episodes.
• internal/api/handlers/sync_test.go - Add handler tests for 200 not_started and 404 episode not found behavior.
• docs/designs/cloud-sync-ui-implementation.md - Document the new sync status endpoint contract.
• docs/designs/episode-qa-checks-mcap-integrity.zh.html - Document the updated episode detail quality-check result modal behavior.

Added Files

• None

Deleted Files

• None

---

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update (documentation changes only)
• Refactoring (code improvement without functional changes)
• Performance improvement (code changes that improve performance)
• Test changes (adding, modifying, or removing tests)

---

Impact Analysis

Breaking Changes

None

Backward Compatibility

Existing synced episodes still return the latest real sync log. Existing episodes with no sync logs now return 200 status=not_started instead of 404; clients that treated no-sync-log 404 as normal should handle this response as the same business state.

---

Testing

Test Environment

• Local Go test environment
• Swagger regenerated with go run github.com/swaggo/swag/cmd/swag@v1.16.6 init -g internal/server/server.go -o docs

Test Cases

• Unit tests pass locally
• Integration tests pass locally
• E2E tests pass (if applicable)
• Manual testing completed

Manual Testing Steps

Not performed.

Test Coverage

• New tests added
• Existing tests updated
• Coverage maintained or improved

Commands run:

go test ./internal/api/handlers -run 'TestGetSyncStatus|TestListEpisodeSyncSummariesGroupsByEpisode' -v

---

Screenshots / Recordings

Not applicable.

---

Performance Impact

• Memory usage: No change
• CPU usage: No meaningful change
• Throughput: No change
• Lock contention: No change

---

Documentation

• README.md updated
• ARCHITECTURE.md updated
• CONTRIBUTING.md updated
• API documentation updated
• Design docs updated in docs/designs/
• No documentation changes needed

---

Related Issues

• Related to episode detail sync status 404 handling

---

Additional Notes

• not_started is a virtual response status for this endpoint and is not inserted into sync_logs.
• 404 now means the episode is missing or soft-deleted.

---

Reviewers

---

Notes for Reviewers

• Please review the endpoint contract in internal/api/handlers/sync.go.
• Please confirm not_started is acceptable as a virtual status for existing episodes with no sync log.

---

Checklist for Reviewers

• Code changes are correct and well-implemented
• Tests are adequate and pass
• Documentation is updated and accurate
• No unintended side effects
• Performance impact is acceptable
• Backward compatibility maintained (if applicable)