Feature/job state model

[Rhea-15]

Summary
Defines a standardised Job interface and JobStatus state machine with enforced forward-only transitions, per-stage error tracking and typed timestamps.

Issue ticket number and link
Closes #18

Changes

• src/server/types/queue.ts — added Job interface with startedAt, completedAt, stageErrors; added JobStatus, PipelineStage types; added isValidTransition() to enforce queued → processing → processed | failed (retry resets failed → queued only); added JobView serialisable shape and toJobView() helper; kept original QueueJob alias for backwards compat
• src/server/services/queue/queue.ts — internal store switched to Map<string, Job>; updateStatus() guards every transition via isValidTransition() and logs a warning on invalid attempts; sets startedAt on first processing entry and completedAt on terminal states; records per-stage errors in stageErrors; retry() only succeeds from failed state; added list(), listViews() with optional ?status= filter
• src/app/api/screenshots/route.ts — GET now returns job list via queue.listViews() with optional ?status= query param filter; POST unchanged except returns typed job.status
• src/app/api/screenshots/[jobId]/route.ts — returns full JobView including startedAt, completedAt, stageErrors
• src/server/runtime.ts — updated queue instantiation to use new InMemoryQueue signature

Testing
[x]Tested locally

• GET /api/screenshots returns [] on fresh start
• POST /api/screenshots with a file returns {"jobId": "job_...", "status": "queued"}
• GET /api/screenshots/[jobId] returns full JobView shape with stageErrors: {}
• GET /api/screenshots?status=queued returns the job correctly
• GET /api/screenshots?status=processed returns [] when no completed jobs exist
• Ran a manual transition test via a temporary route confirming: startedAt is set on processing, completedAt is set on both processed and failed, stageErrors records per-stage error detail, invalid transition (processed → queued) was blocked and state remained processed
• npm run backend:check, npm run lint, npm run build all pass

Checklist before requesting a review

[x]Code follows the project's TypeScript style conventions
[x] No secrets or .env values are committed
[x]I have performed a self-review of my code
[x]CI passes

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[madhav2348]

@Rhea-15 please resolve the merce conflict,

[Rhea-15]

@madhav2348 done resolving the merge conflicts