Add SSE-based notification center for background job events (#1496) - #2057

Summary

Closes #1496 (v1 slice — see scope notes below).

Adds a preview-gated, per-user real-time notification center. Today, background jobs (Tools Service, e.g. AI OCR) only report progress to a client with an open /jobs/:jobId/progress SSE connection — closing the tab or navigating away silently loses the completion event. This adds a small, durable record of terminal job events that survives across page loads and multiple open tabs.

• server/services/notifications/NotificationService.js — in-process event bus (notify(userId, type, data)); persists then broadcasts to any live SSE listeners.
• server/services/notifications/NotificationStore.js — per-user JSON-file persistence (contents/data/notifications/<userId>.json, capped at 200 entries/user).
• server/routes/notifications.js — GET /api/notifications/stream (SSE, multi-tab per user), GET /api/notifications (list), POST /:id/read, POST /read-all. Gated behind a new notifications feature flag (default off) + migration V078.
• server/routes/toolsService/jobStore.js — notifyClients() now also emits a notification on terminal job states only (completed/error/cancelled) — intermediate progress ticks are intentionally not persisted (too frequent, e.g. one per OCR page, to be a useful notification-list entry).
• client/src/features/notifications/ — useNotifications hook (REST initial load + SSE live updates, same fetch+ReadableStream pattern as useEventSource.js for custom auth headers) and NotificationBell/NotificationPanel UI, mounted in Layout.jsx's header for authenticated users when the flag is enabled.

Deliberately out of scope for this slice (per the issue's own implementation-plan comment)

• Durable, multi-instance storage — the original design assumed PostgreSQL ([Platform] PostgreSQL + persistence abstraction layer #1490 /[Platform] Multi-instance horizontal scaling via PostgreSQL LISTEN/NOTIFY #1499 ), which doesn't exist in this codebase yet. This ships a simple per-user JSON-file store behind the same NotificationService.notify() call sites, so swapping to a real DB later doesn't require touching producers.
• Other event types — only background-job lifecycle events are wired up. chat.shared (no such feature exists yet) and system.announcement (no admin broadcast UI) have no producer in this PR.
• Toast/desktop notifications — only the bell + panel; no Notification Web API integration.

Full write-up: docs/notifications-feature.md.

Test plan

• npm run lint:fix && npm run format:fix — clean on all changed files
• New unit tests: server/tests/NotificationStore.test.js (append/list/scope-per-user/mark-read/mark-all-read/path-traversal-rejection/200-item cap) — all pass
• Verified server/routes/notifications.js, server/routes/toolsService/jobStore.js, and server/featureRegistry.js import cleanly with no errors
• Manual end-to-end verification (start dev server, enable the feature flag, trigger an OCR job, confirm the bell updates) — not done in this session (no running dev environment with a configured OCR provider); would appreciate a manual smoke test before merge

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https://claude.ai/code/session_01A8KD83dJU7XCZx1GUDD1kk

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