feat: name the stack so OpenTaberna ranks for FastAPI, Python and Angular commerce

[PhilippTheServer]

Closes #6

Adds a What it runs on / Worauf es läuft section to both pages and a Stack section to llms.txt, naming what the repos actually use (checked against fastapi/pyproject.toml and docker-compose.yml): FastAPI, async SQLAlchemy, PostgreSQL, Redis, Keycloak, Angular, Stripe, DHL, Paperless-ngx, MinIO, Prometheus, Grafana, OpenTelemetry, Docker Compose. That targets the niche searches the big platforms do not hold (FastAPI e-commerce, Python headless shop, Angular storefront, self-hosted shop with Keycloak).

check_site.rb fails if any of the three loses the section or one of those terms, and the English section must keep its place in the section order. README documents it.

Verification

./scripts/verify.sh (Ruby 3.3 container)

before the content existed:
check_site.rb: 40 failure(s)
  ✗ index.html: has lost the "What it runs on" section
  ✗ index.html: stack section has lost "FastAPI"
  ✗ index.html: stack section has lost "PostgreSQL"
  …

after:
check_site.rb: all checks passed
==> all verification passed

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