Add plain-text report summary export and frontend sharing actions

[DeepZone]

Motivation

• Make RouteForge results easy to copy into change-tickets, emails or maintenance docs by providing a short, ticket-ready plain-text summary and better export/download headers.
• Keep behavior read-only and non-breaking while enabling frontend copy/download flows for Markdown, HTML and the new plain summary.

Description

• Backend: added render_plain_summary(report_json: dict) in backend/app/services/report_renderer.py to produce defensive, human-readable plain-text summaries (unknown fallbacks, holder from details.resource_holder.holder, optional preflight decision, recommendations list).
• Backend API: added GET /api/reports/{report_id}/summary and updated /markdown and /html responses to set Content-Type with charset=utf-8 and Content-Disposition attachment filenames for downloads in backend/app/api/routes_reports.py.
• Frontend: added requestText helper and API functions getReportMarkdown, getReportHtml, getReportSummary in frontend/src/api.ts, and implemented export actions in ReportView and the Reports list (Copy Summary, Copy Markdown, Download Markdown, Download HTML, Open) with clipboard usage and unobtrusive feedback in frontend/src/components/ReportView.tsx and frontend/src/App.tsx.
• Docs: documented the feature in README.md under “Export and sharing” and noted it in RELEASE_NOTES.md.
• Tests: extended backend/tests/test_api_smoke.py with test_report_export_endpoints to validate the summary endpoint and export content-types.

Testing

• Ran backend test suite with cd backend && pytest -q and all tests passed (58 passed).
• Built frontend with cd frontend && npm run build and the production build completed successfully.
• New smoke test test_report_export_endpoints verifies GET /api/reports/{id}/summary returns 200, includes RouteForge and Status: in the body, and that markdown/html endpoints return proper text/markdown / text/html content-types.

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Codex Task