fix: complete the docs/→design/ and openapi→api/ rename references

The file moves (docs/ → design/, docs/api/openapi.json → api/openapi.json) landed
in d01a8fd, but the matching reference updates did not — so mgmt.rs's drift-test
`include_str!("../../../docs/api/openapi.json")` pointed at a path that no longer
exists and the host failed to build. This restores it and updates every reference:

  - mgmt.rs include_str! → ../../../api/openapi.json (fixes the build)
  - web/orval.config.ts codegen target, web/Dockerfile, .dockerignore
  - deb/rpm/Arch packaging install paths
  - CLAUDE.md, the .gitea CI workflows, code doc-comments, design-doc cross-links

docs-site route URLs (/docs/...) untouched.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packaging/debian/build-deb.sh

@@ -57,7 +57,7 @@ install -Dm0644 scripts/headless/punktfunk-sink.conf   "$SHAREDIR/headless/punkt
 install -Dm0644 scripts/host.env.example           "$SHAREDIR/host.env.example"
 install -Dm0644 packaging/bazzite/host.env         "$SHAREDIR/host.env.bazzite"
 install -Dm0644 packaging/kde/host.env             "$SHAREDIR/host.env.kde"
-install -Dm0644 docs/api/openapi.json              "$SHAREDIR/openapi.json"
+install -Dm0644 api/openapi.json              "$SHAREDIR/openapi.json"
 install -Dm0644 LICENSE-MIT                         "$DOCDIR/LICENSE-MIT"
 install -Dm0644 LICENSE-APACHE                      "$DOCDIR/LICENSE-APACHE"
 install -Dm0644 README.md                           "$DOCDIR/README.md"