Prepare v0.3.0-alpha: version bump and demo / release-readiness docs

[DeepZone]

Motivation

• Prepare a public alpha demo and release-ready repository state for v0.3.0-alpha with clear onboarding and demo artifacts.
• Surface the new user-facing version and package/runtime version strings across frontend and backend UIs and packages.
• Provide guided demo flows, screenshot capture plan, release notes, roadmap and contributor guidance without changing domain logic or adding any write operations.

Description

• Bumped user-facing version to v0.3.0-alpha and package/runtime version to 0.3.0 in backend/app/api/routes_system.py, backend/app/main.py, backend/pyproject.toml, frontend/package.json, frontend/package-lock.json, and updated UI strings in frontend/src/App.tsx and frontend/src/components/Layout.tsx.
• Rewrote README.md for a 60-second onboarding (pitch, quickstart with docker compose, demo mode instructions, URLs, core checks, result model, export/sharing, and limitations).
• Added structured demo and release artifacts: docs/demo-flow.md, docs/releases/v0.3.0-alpha.md, and a screenshot capture plan in docs/screenshots/README.md (no placeholder images created).
• Created project guidance files ROADMAP.md and CONTRIBUTING.md, and updated RELEASE_NOTES.md with the v0.3.0-alpha section.
• Ensured .env.example contains ROUTEFORGE_DEMO_MODE and common vars and kept all changes non-functional (no new external data sources, no write ops, no API-breaking changes to endpoints).

Testing

• Ran backend test suite with cd backend && pytest -q, result: 61 passed (with warnings about deprecations).
• Built the frontend with cd frontend && npm run build, result: production build succeeded (vite build completed; npm emitted a non-blocking http-proxy env config warning).
• No functional tests failed and no domain logic or write operations were added.

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Codex Task