fix(deploy): align Cloud deployment contract

[sunormesky-max]

Description

Aligns the Cloud deployment contract across Docker Compose, the standalone Kubernetes manifest, and the Helm chart.

• Requires the Cloud EPICODE_API_KEY and injects all frontend production runtime variables.
• Routes /api/v1/... to backend /v1/... consistently, preserving frontend /api/trpc and adding compatible Cloud aliases for health, registration, and public stats.
• Restricts application services to internal networking, adds frontend and Redis probes, and keeps the gateway/Ingress as the public entry point.
• Adds CI-backed deployment configuration validation and updates deployment documentation.

Fixes # (issue)

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• Documentation update

How Has This Been Tested?

• Unit tests
• Integration tests
• Manual testing

Please describe the test configuration:

• python scripts/validate-deployment-config.py
• YAML parsing for Compose, Kubernetes, and Helm values
• cargo fmt --manifest-path backend/Cargo.toml --all -- --check
• cargo check --manifest-path backend/Cargo.toml --bin epicode-cloud --locked
• cargo test --manifest-path backend/Cargo.toml --bin epicode-cloud --locked
• npm run check, npm run lint, and npm run test in frontend
• Production frontend /health endpoint against a bundled server

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

Screenshots (if applicable)

N/A

Additional Notes

On Windows, the existing npm run build script's POSIX shell quoting breaks only its esbuild banner argument after Vite completes. The equivalent esbuild bundle command and the production health endpoint both passed; the container build continues to use Linux shell semantics.