Add opt-in search indexing middleware

[crack-kitty]

Summary

This replaces the original kitty-test approach with a safer router-level opt-in model for search indexing.

The kitty-test branch tried to make indexing configurable by moving X-Robots-Tag out of default-headers into a separate noindex middleware, then adding per-service middleware env vars. That inverted OnRamp's existing safety model: routes that still used only default-headers, especially file-provider/external routes and other unconverted edge paths, could silently lose the noindex header and become indexable by default. It also made examples risky because replacing a middleware chain with only default-headers could drop functional middleware such as redirects, compression, buffering limits, or other route-specific behavior.

This PR keeps the existing architecture intact:

• default-headers continues to send the current noindex X-Robots-Tag
• Traefik entrypoint defaults still protect routes that do not define router-level middleware
• allow-indexing-headers clears X-Robots-Tag only when explicitly appended after default-headers
• Docker service routers get middleware env overrides whose defaults preserve current behavior
• existing functional middleware defaults are retained, including MinIO gzip, Pi-hole redirects, and upload-size limits
• scaffolded services default to default-headers@file and can opt in by appending allow-indexing-headers@file
• docs cover the ordering requirement and warn against removing functional middleware
• validation checks ensure services do not allow indexing by default and that websecure routers expose middleware overrides

Validation

• python3 make.d/scripts/check-search-indexing.py
• git diff --check
• targeted yamllint on changed core YAML and representative service YAML

Notes

Full yamllint -c .yamllint services-available is currently blocked by a pre-existing duplicate healthcheck key in services-available/jellyfin.yml. This PR intentionally leaves Jellyfin unchanged so that can be handled as a separate main-branch cleanup.