test: build comprehensive automated test suite

[bakaxbaka]

What this adds

• Shared Flask pytest fixtures
• API route and validation contract tests
• Deterministic secp256k1/ECDSA service round-trip tests
• Signature-malleability regression tests
• Mocked blockchain network/service tests
• Application factory, error-handler, security-header, and HTML route tests
• Frontend/API-client contract tests
• Dedicated development requirements with pytest + coverage
• Cross-platform GitHub Actions CI for Python 3.11/3.12 on Ubuntu and Python 3.11 on Windows
• Dependency checks, Python compilation checks, JUnit results, and coverage artifacts

The workflow is intentionally network-independent at test time: external blockchain calls are mocked.