Real-time progress streaming for bulk jobs (SSE)

[Aditya8369]

Implemented real-time progress streaming for bulk jobs via SSE (server-sent events).

What changed
Backend: SSE route
backend/routes/verification.js
Added:
GET /api/verification/bulk/:jobId/events/stream
Protected with protect + adminOnly
Backend: SSE controller
backend/controllers/verificationController.js
Added streamBulkJobEvents(req, res)
SSE headers set:
Content-Type: text/event-stream
Cache-Control: no-cache, no-transform
Connection: keep-alive
Emits events:
event: heartbeat (initial + periodic)
event: jobStatus with { status, totalRows, processedRows, successCount, failureCount }
event: rowResult for newly-available rows (derived from job.results up to processedRows)
event: done when status becomes completed or failed
event: error on failure
Polls BulkVerificationJob.findById(jobId) until done.
Backend test
backend/tests/verificationBulkSse.test.js
Validates SSE framing and that jobStatus + done events appear.

closes #447

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