feat: claim status notifications with permission UX, polling backoff, and SSE

[darcszn]

Closes #94

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Summary

Implements optional browser notifications and in-app toasts for claim status changes on watched claims.

Changes

Frontend

File	Change
useNotificationPermission.ts	One-time permission request helper — localStorage flag prevents re-prompting after denial or dismissal
useClaimWatcher.ts	Polls /api/claims/status with exponential backoff (base 15 s, cap 60 s); pauses when tab is hidden (Page Visibility API); resumes immediately on tab focus
useNotifications.ts	Privacy-safe lock-screen copy; in-app toast fallback when permission not granted; per-claim mute() / snooze() controls; new useClaimStatusNotifications hook
NotificationPermissionBanner.tsx	One-time banner with clear value explanation; ClaimNotificationsToggle for full feature disable
ClaimsBoard.tsx	Wires banner, toggle, watcher, and status-change notifications

Backend

File	Change
claims.controller.ts	GET /api/claims/status (polling, throttled 30 req/min); GET /api/claims/status/stream (SSE with 25 s heartbeat, throttled 5 conn/min)
claims.service.ts	getClaimStatuses(), subscribeToStatusChanges(), publishStatusChange() (static, call from indexer/queue)

Docs

• docs/claim-notification-latency.md — latency budget table, agreed SLO, battery impact, indexer lag, multi-instance note

Acceptance Criteria

• Users who deny permission are not re-prompted on subsequent visits
• Status changes surface within agreed max delay (SLO: 2 min, see latency doc)
• Feature can be fully disabled via ClaimNotificationsToggle
• Notification copy does not leak sensitive details on shared/locked screens
• Polling pauses when tab is hidden (battery impact considered)
• Exponential backoff prevents tight retry loops on failure

Notes

• The SSE pub/sub uses an in-process listener registry. For multi-instance deployments, replace ClaimsService.statusListeners with a Redis pub/sub channel (noted in code and docs).
• ClaimsService.publishStatusChange() is the integration point for the indexer — call it whenever a claim status changes in the DB.

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