pattern: add Datadog API key detection

[Rome-1]

What

Add a secret detection pattern for Datadog API keys to both Node and Python implementations.

Pattern info

• Service: Datadog (monitoring/observability)
• Format: 32-character hex string, typically prefixed with context like DD_API_KEY=
• Regex suggestion: /\b[a-f0-9]{32}\b/ with contextual anchoring (e.g., datadog|DD_API_KEY|DD_APP_KEY)

Files to change

1. node/src/scanners/secret-patterns.ts — add pattern to the DEFAULT_SECRET_PATTERNS array
2. python/rafter_cli/scanners/secret_patterns.py — add identical pattern
3. node/tests/secret-patterns.test.ts — add test with fake credential
4. python/tests/test_regex_scanner.py — add matching test

How to test

cd node && pnpm test -- --grep "datadog"
cd python && pytest tests/test_regex_scanner.py -k "datadog"

Notes

• Follow the existing pattern format: { name, severity, regex, description }
• Both implementations must have identical regex — see existing patterns for examples
• Use realistic but fake test credentials