fix: implement pagination/filtering/sorting standards (#257) and background job processing (#258)

[boss572]

Summary

Closes #257, Closes #258

This PR implements two backend enhancements: cursor-based pagination standards for all list endpoints, and a general-purpose background job processing system with retry and dead-letter queues.

Issue #257 — Cursor-Based Pagination, Filtering & Sorting

New Files

• backend/src/utils/pagination.ts — Core types (PaginationParams, PaginationMeta, ListFilters), helpers (encodeCursor, decodeCursor, buildPaginationMeta, parsePaginationParams, parseFilters), and sort-field allowlist registry
• backend/src/middleware/schemas/paginationSchemas.ts — Joi validation for limit, cursor, sort, order, and filter params
• backend/src/middleware/pagination.ts — Express middleware that validates & attaches req.pagination + req.listFilters

Modified Files

• backend/src/controllers/EnrollmentController.ts — getUserEnrollments and getCourseEnrollments now use cursor-based pagination with next_cursor/total_count/has_more response metadata
• backend/src/docs/openapi.ts — Added CursorPagination schema

Standard Query Parameters

?limit=20&cursor=...&sort=enrolledAt&order=desc&status=active&type=course

Standard Response Metadata

{
  "pagination": {
    "next_cursor": "base64url-encoded-value",
    "total_count": 250,
    "has_more": true
  }
}

Issue #258 — Background Job Processing

New Files

• backend/src/services/jobQueue.ts — Redis-backed job queue with persistent storage, typed Redis client, retry with exponential backoff (1s → 5min cap), dead-letter queue, enqueue/getJob/getStats/listJobs/retryDeadLetter/removeJob
• backend/src/controllers/jobController.ts — 8 endpoints: stats, list/filter jobs, job detail, job progress, retry dead-letter, remove job, enqueue job, list dead-letter
• backend/src/routes/jobRoutes.ts — REST routes at /api/jobs

Modified Files

• backend/src/index.ts — Job queue initialized at startup with 8 built-in handler types, routes mounted at /api/jobs and /api/v1/jobs, graceful shutdown integration
• backend/src/docs/openapi.ts — Added Job, JobStats, EnqueueJobRequest schemas + Jobs tag

Supported Job Types

email, credential_minting, analytics_aggregation, notification, report_generation, data_export, content_processing, general

API Endpoints

Method	Path	Description
GET	/api/jobs/stats	Aggregate job statistics
GET	/api/jobs	List jobs (filterable, paginated)
POST	/api/jobs	Enqueue a new job
GET	/api/jobs/:id	Get job details
GET	/api/jobs/:id/progress	Get job progress
POST	/api/jobs/:id/retry	Retry dead-lettered job
DELETE	/api/jobs/:id	Remove a job
GET	/api/jobs/dead-letter	List dead-lettered jobs

TypeScript

Typecheck passes with 0 new errors. The only pre-existing error is an unrelated graphqlBootstrap reference on line 549 of index.ts.

Files Changed

9 files changed, 1321 insertions(+), 47 deletions(-)
backend/src/controllers/EnrollmentController.ts   (modified)
backend/src/controllers/jobController.ts           (new)
backend/src/docs/openapi.ts                         (modified)
backend/src/index.ts                                (modified)
backend/src/middleware/pagination.ts                (new)
backend/src/middleware/schemas/paginationSchemas.ts (new)
backend/src/routes/jobRoutes.ts                     (new)
backend/src/services/jobQueue.ts                    (new)
backend/src/utils/pagination.ts                     (new)