Sprint v0.4.0-alpha: Add Data Source Diagnostics (backend + UI + tests)

[DeepZone]

Motivation

• Provide transparent, per-check diagnostics about which external data sources were queried and why a check may be UNKNOWN/WARNING (timeouts, parse errors, empty data, cache hints, etc.).
• Make diagnostics available in reports and the UI so operators can distinguish data problems from true operational issues.
• Preserve existing public APIs and demo-mode behavior while keeping the system read-only.

Description

• Added a centralized diagnostics model backend/app/core/source_diagnostics.py with status constants and make_source_diagnostic(...) to produce a stable diagnostic shape.
• Instrumented RipeStatClient (get_with_diagnostics(...)) to measure duration, honor demo mode, use existing caching best-effort, and classify errors (TIMEOUT, RATE_LIMITED, HTTP_ERROR, PARSE_ERROR, EMPTY_RESPONSE, OK, etc.) while keeping get(...) backwards-compatible.
• Propagated diagnostics into checks: Prefix, ASN (including aggregated RPKI-batch metadata), and Preflight now include details.source_diagnostics and where appropriate attach per-call source_diagnostic entries from RPKI checks.
• Improved routing-visibility UNKNOWN explanations when the RIPEstat routing-status payload cannot be interpreted and flagged the structure as unknown.
• Extended report templates (report.md.j2, report.html.j2) with a Data Source Diagnostics section and added a Diagnostics table/card to the frontend ReportView with a new SourceDiagnostic type in frontend/src/types.ts.
• Added tests (backend/tests/test_source_diagnostics.py) and enhanced smoke tests to assert details.source_diagnostics presence; bumped visible version strings to v0.4.0-alpha / 0.4.0.

Testing

• Ran backend unit test suite with cd backend && pytest -q resulting in all tests passing (64 passed).
• Ran frontend build with cd frontend && npm run build and the production build completed successfully.
• Added backend/tests/test_source_diagnostics.py and extended existing smoke tests to check that details.source_diagnostics is present for Prefix, ASN and Preflight checks, and demo-mode diagnostics indicate demo_mode or the demo message.

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Codex Task