fix(backend): add cursor pagination to enrollment and queue list endpoints

[jajafwangshak86-ops]

Summary

Closes #182

Adds cursor-based pagination to the three unbounded list endpoints identified in the issue, preventing multi-MB responses on busy deployments.

Changes

New file: backend/src/utils/pagination.ts

• paginate<T>(items, opts) helper that slices an array by limit + cursor offset
• Uses the SDK's canonical encodeCursor/decodeCursor (format: base64("0:index")) so cursor semantics are shared across the stack
• Default limit 50, hard cap 200, invalid cursor throws ValidationError (→ 400)

backend/src/routes/enrollments.ts

• GET /api/enrollments/:identity — accepts ?limit and ?cursor, returns { items, nextCursor, total }
• GET /api/enrollments/queue/:queueId — same pagination contract

backend/src/routes/queues.ts

• GET /api/queues — accepts ?limit, ?cursor, and existing ?status filter; returns { items, nextCursor, total }

Before / After response shapes

Before (unbounded):

GET /api/enrollments/queue/sneaker-drop-001
→ [ { ... }, { ... }, ...50000 records ]

After (paginated):

GET /api/enrollments/queue/sneaker-drop-001?limit=50
→ {
    "items": [ ...50 records ],
    "nextCursor": "MDox",
    "total": 50000
  }
GET /api/enrollments/queue/sneaker-drop-001?limit=50&cursor=MDox
→ { "items": [ ...next 50 ], "nextCursor": "MDoy", "total": 50000 }

Backward compatibility

Omitting limit and cursor returns the first 50 items in the paginated envelope — callers that previously iterated a bare array will need to read .items, but no params → no breakage at the transport level.

Tests

• Fixed stale assertions in enrollments.route.test.ts that expected bare arrays
• Fixed queues.route.test.ts cursor values to use the correct encodeCursor(0, N) format and error shape (res.body.error.message)
• Added multi-page traversal tests for both enrollment endpoints (3 records, limit 2 → page 1 returns 2 items + cursor, page 2 returns 1 item + null cursor)
• All 156 backend tests pass

Docs

• docs/api-reference/enrollments.md — updated response schema with pagination envelope and query param table
• docs/api-reference/queues.md — updated to show paginated envelope and full query param table

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