chore: polish .env.example and report template language

[DeepZone]

Motivation

• Remove duplicate environment keys and tidy .env.example for clearer release/readme guidance.
• Bring report templates to consistent operational English to match UI and documentation language.
• Add lightweight regression guards so the cleanup does not regress in future changes.

Description

• Removed duplicate alert webhook keys and reorganized .env.example into logical sections (Core/App, Database, RIPEstat, RPKI Provider, BGP Visibility, Change/Post-Change, Alerts/Webhooks, Security/CORS) while keeping defaults.
• Replaced German visible labels/headings in backend/app/templates/report.md.j2 with English equivalents (for example Eingabe → Input, Gesamtbewertung → Overall Assessment, Rohdaten → Raw Data).
• Replaced German visible labels/headings in backend/app/templates/report.html.j2 with English equivalents and adjusted table/summary labels (for example Quelle → Source, Dauer → Duration, Ein-/ausklappen → Expand/collapse).
• Added backend/tests/test_release_cleanup_guards.py with small tests that assert .env.example has no duplicate keys and that both report templates contain the expected English headings.

Testing

• Ran backend test suite with cd backend && pytest -q which passed (107 passed) and included only non-blocking deprecation warnings.
• Ran frontend build with cd frontend && npm run build which completed successfully (Vite production build).
• The added tests were included in the backend run and passed as part of the pytest output.

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Codex Task