RIPEstat: add retry/resilience, stale-cache fallback and diagnostics (v0.4.2-alpha)

[DeepZone]

Motivation

• Improve robustness when querying RIPEstat so temporary network/HTTP errors do not immediately fail checks.
• Make retry behaviour configurable and expose retry/fallback information in Data Source Diagnostics so operators can see why data is stale.
• Provide an optional stale-cache fallback to continue serving results when live queries fail while keeping demo mode single-attempt and preserving backward cache compatibility.

Description

• Added new configuration settings in backend/app/config.py and .env.example: ripestat_timeout_seconds, ripestat_max_retries, ripestat_retry_backoff_seconds, and ripestat_use_stale_cache_on_error, with sensible defaults.
• Extended make_source_diagnostic(...) in backend/app/core/source_diagnostics.py to include optional fields retry_count, attempts, fallback_used, fallback_reason, and stale_cache_used.
• Reworked RipeStatClient.get_with_diagnostics(...) in backend/app/services/ripe_stat_client.py to implement timeout/retry/backoff logic using the new settings, retry only on timeout/502/503/504/transient network errors, classify HTTP 429 as RATE_LIMITED (capturing Retry-After when present), and optionally fall back to stale cache entries (keeping backward compatibility for older cache shapes).
• Kept demo mode behavior unchanged (no retries, attempts=1, retry_count=0, fallback_used=false, freshness=LIVE).
• Frontend changes: extended SourceDiagnostic type in frontend/src/types.ts and updated ReportView (frontend/src/components/ReportView.tsx) to display Attempts, Retries, Fallback and Stale Cache, plus a small warning UI when a fallback was used and a rate-limit hint when status is RATE_LIMITED.
• Report templates updated (backend/app/templates/report.md.j2 and backend/app/templates/report.html.j2) to include the new diagnostics columns and a note when cached fallback data was used.
• Documentation and metadata: added Retry and Resilience section to README.md, added v0.4.2-alpha entry to RELEASE_NOTES.md, and bumped versions to v0.4.2-alpha (UI) / 0.4.2 (packages/runtime).

Testing

• Ran backend unit tests with cd backend && pytest -q, which passed (73 passed, warnings unrelated to this change).
• Built the frontend with cd frontend && npm run build, which completed successfully and produced a production build.
• Added/updated automated tests covering retry and fallback scenarios in backend/tests/test_ripe_stat_client.py (timeout->success, 503->success, 404 no-retry, 429 rate-limited with Retry-After, timeout + stale cache fallback, timeout without cache, and demo-mode behavior) and a test for the diagnostic shape in backend/tests/test_source_diagnostics.py, and adjusted smoke tests to assert the new attempts diagnostic field; all automated tests passed.

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Codex Task