Support configurable base path for STAC and other EOAPI components

[achtsnits]

Right now, the EOAPI stack assumes certain fixed paths for routing—for example, the STAC API is expected to be available under /stac.

In some cases, like on DLR's Terrabyte platform, we need more flexibility—for instance, serving STAC under /api instead. While it's possible to handle this with rewrites or redirects at the ingress level, the application itself still uses hardcoded paths for generated links and doesn’t fully support custom routes.

This likely isn’t something that can be solved in the Helm chart (i.e. within this code repo) alone, as it also depends on how routes are handled in the application code, ...