Add cache & freshness transparency (v0.4.1-alpha)

[DeepZone]

Motivation

• Improve observability of whether external data was served live or from cache and provide operators with cache age/TTL/freshness so preflight decisions can be assessed.
• Support demo-mode semantics and prepare internal force_refresh support without breaking existing read-only behavior or APIs.

Description

• Add make_cache_metadata(...) in backend/app/core/source_diagnostics.py to return cache metadata and compute freshness as one of LIVE|FRESH|EXPIRING_SOON|STALE|UNKNOWN following the specified rules.
• Extend make_source_diagnostic(...) to optionally include cache_ttl_seconds, fetched_at, expires_at, and freshness while preserving existing fields and shape.
• Enhance RipeStatClient.get_with_diagnostics(...) in backend/app/services/ripe_stat_client.py to store and return minimal cache metadata (wrapped payload, fetched_at, ttl_seconds, cache_key) in a backwards-compatible way, to compute cache_age_seconds/cache_ttl_seconds/freshness, and to accept force_refresh: bool (bypasses cache and records details.force_refresh). Demo mode now returns cached=false, freshness=LIVE, message='Demo data returned', and details.demo_mode=true.
• Introduce RIPESTAT_CACHE_TTL_SECONDS configuration (default 900) via settings and add it to .env.example and backend/app/config.py.
• Update frontend types (frontend/src/types.ts) and ReportView (frontend/src/components/ReportView.tsx) to display Freshness, Cache HIT/MISS, Age and TTL with color hints and a formatDuration helper for s/m/h display.
• Extend Markdown and HTML report templates to include Freshness, Cache Age and TTL with Unknown fallbacks.
• Add/adjust tests: extended backend/tests/test_source_diagnostics.py, new backend/tests/test_ripe_stat_client.py, and added smoke assertions in backend/tests/test_api_smoke.py to ensure freshness is present in diagnostics.
• Bump user-facing and package versions to v0.4.1-alpha / 0.4.1 and update README/RELEASE_NOTES with a short Cache & Freshness section.

Testing

• Ran backend test suite with cd backend && pytest -q, all tests passed (67 passed).
• Built frontend with cd frontend && npm run build, build completed successfully.
• Added automated tests verifying make_cache_metadata(...) freshness rules and RipeStat client demo/cache diagnostics which passed as part of the test run.

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Codex Task