2.0.19: Add versionless GP247 ecosystem fingerprint

Emit a brand marker on every response so technology-detection services
(Wappalyzer, BuiltWith, W3Techs) can recognise GP247-powered sites:

- HTTP header `X-Powered-By-GP247: GP247` on all responses
- `<meta name="generator" content="GP247">` injected into HTML documents

Implemented as a global middleware in core so it survives core-only sites
(no storefront) and any active/custom template alike, instead of living in
a swappable front-end view. The marker is a constant brand string with no
version, so it cannot be used to target version-specific vulnerabilities.
Opt-out via config gp247.fingerprint (env GP247_FINGERPRINT=false).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>