Fix compatibiity with calibre-web and standard ebooks

[mawise]

Fix OpenSearch descriptor fetching and resolution

The OPDS proxy previously attempted to fetch the OpenSearch descriptor at search time rather than during feed parsing. This failed on OPDS servers like calibre-web that required proxy authentication.

This commit shifts the OpenSearch descriptor fetching into the feed parsing pipeline (in serveAtom), so that the true search template URL is injected directly into the proxy's rendered HTML. It correctly fetches the OSD files passing along any required credentials or custom User-Agents configured for that feed.

Additionally, standard OPDS search queries are now cleanly supported by simply appending ?query= to non-OSD endpoints.

fix: support generic xml content types for opds feeds

Standard Ebooks serves their OPDS search results with a generic application/xml or text/xml content type rather than the standard application/atom+xml. This causes opds-proxy to treat the results as a regular file download instead of intercepting and rendering them via HTML templates.

This commit updates the FormatByMimeType mapping in formats.go to recognize application/xml and text/xml as ATOM feeds, ensuring these feeds are correctly processed and rendered. Actual book downloads (e.g., application/epub+zip) are unaffected.

Tests have been added to verify that FormatByMimeType correctly handles both standard and generic XML content types, as well as book download formats.

Update OpenSearch template parsing to prioritize acquisition types

Update ParseOpenSearchTemplate to first prefer templates with type="application/atom+xml;profile=opds-catalog;kind=acquisition". If not found, it falls back to standard OPDS types, then general Atom types, and finally any valid template as a last resort. Update tests accordingly to reflect the new priority.

fix(opds): use prefix match for acquisition links

Updates the link parsing logic to use strings.HasPrefix when checking the link relationship for acquisitions, supporting extensions like /open-access used in OPDS 1.2 tags. Also updates a test case to verify this behavior.

[mawise]

Hi,

I was excited when I came across your project! I had a couple issues fetching my authenticated calibre-web opds feed, and with the standard ebooks feed, so I resolved the incompatibilities. These changes are AI-assisted. I've got it running great on my fork, deployed and working for both opds sources.

No pressure to merge these changes if you don't want to. Thanks for your work!