Prepare RouteForge v0.1.0-alpha: CI, README and demo-mode UI

[DeepZone]

Motivation

• Make the project demonstrable and reproducible as v0.1.0-alpha by hardening CI, surfacing demo-mode in the UI and docs, and adding concise usage examples.
• Ensure the frontend build step in CI works reliably both with and without a lockfile and keep backend tests running on Python 3.12.
• Make the demo mode clearly visible to avoid accidental usage of example data in production scenarios.

Description

• Add robust GitHub Actions workflow .github/workflows/ci.yml that runs backend tests with Python 3.12 and builds the frontend with Node 22, using npm ci when package-lock.json exists and npm install otherwise.
• Improve the frontend ReportView demo-mode banner to a more prominent warning which is only shown when report.details.demo_mode === true.
• Update README.md to RouteForge v0.1.0-alpha, add a Typische Nutzung section with three concrete curl examples (prefix check, ASN check, ASN-RPKI batch), clarify Demo-Modus usage, and expand the status explanations for OK, WARNING, CRITICAL, and UNKNOWN.
• Verified backend already exposes details.demo_mode for checks and the ASN-RPKI batch status logic meets requirements (CRITICAL > WARNING > OK, UNKNOWN when no prefixes, API errors treated as warnings), so no behavioral changes to existing endpoints were made.

Testing

• Ran backend unit tests with cd backend && pytest -q, which completed successfully (16 passed, 20 warnings).
• Built the frontend with cd frontend && npm run build, which succeeded (production build completed).
• The CI workflow was created to run the same automated steps (backend pytest -q, frontend npm ci/npm install + npm run build).

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Codex Task