feat: API response time logging, delivery failure reasons, config validation, notification sorting

[Lost-Z]

Summary

Implements four issues in a single PR.

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Closes #491 — Add API Response Time Logging

• New ResponseTimeMiddleware (listener/src/middleware/response-time.ts)
  • Records start time per-request via a symbol key on the ServerResponse
  • Sets X-Response-Time header on every response
  • Logs completed requests at INFO; flags requests exceeding slowRequestThresholdMs (default 1 000 ms) at WARN
  • Tracks totalRequests, slowRequests, totalResponseTimeMs, maxResponseTimeMs
  • slowRequestThresholdMs is configurable via EventsServerOptions
• Wired into createEventsServer: start() at request open, finish() at the 404 fallthrough
• New GET /api/metrics/response-time endpoint exposes counters; ?reset=true resets them

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Closes #493 — Add Delivery Failure Reasons

• NotificationSearchService: failureReason field added to NotificationSearchResult
  • Populated from last_error (scheduled_notifications) and error_reason (processed_events)
• Dashboard eventsApi.ts: failureReason exposed on NotificationSearchResult
• NotificationSearchPage: renders a Failure reason row in NotificationResultCard when the field is non-null

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Closes #494 — Improve Configuration Validation

• New validateConfig(config) exported from listener/src/config.ts
  • Validates RPC URL format, port range, poll intervals, scheduler lock/batch settings, retry multiplier ordering, rate-limit minimums, analytics bucket size, cleanup retention floors
  • Collects all violations before throwing so operators see every problem in one error message
  • Throws ConfigError with a numbered list of descriptive messages
• index.ts: validateConfig(config) is called immediately after loadConfig() — invalid configuration blocks startup before any service initialises

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Closes #495 — Add Notification Sorting Options

• New NotificationSortOption type (newest | oldest | priority | delivery_status) added to dashboard/src/types/event.ts; sortBy field added to EventFilters
• sortEvents() utility in eventData.ts implements all four strategies; filterEvents() accepts and applies sortBy
• EventFiltersBar: Sort by dropdown; selection persisted to localStorage via setSortBy so it survives page refreshes
• eventStore: setSortBy action writes to localStorage; loadPersistedSort() restores on init; selectFilteredEvents and useFilteredEvents thread sortBy through
• Server-side: NotificationSearchParams.sortBy (newest | oldest | status) added to NotificationSearchService; sort applied before pagination
• events-server: extracts sortBy query param and forwards to the search service
• NotificationSearchPage: Sort by dropdown (newest / oldest / status); included in filtersKey so page resets on sort change; reset by Clear filters

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