fix(opds): add Stanza rels and clarify legacy cover/thumbnail mappings

Add Lexcycle Stanza rel values to COVER and THUMBNAIL for broader compatibility with legacy OPDS catalogs, and document existing non-standard ManyBooks rels.

- add 'x-stanza-cover-image' (Lexcycle Stanza)
- add 'x-stanza-cover-image-thumbnail' (Lexcycle Stanza)
- clarify 'http://opds-spec.org/cover' as ManyBooks legacy (non-standard)
- clarify 'http://opds-spec.org/thumbnail' as ManyBooks legacy (non-standard)

Author: zeedif

opds.js

@@ -20,11 +20,13 @@ export const REL = {
     GROUP: 'http://opds-spec.org/group',
     COVER: [
         'http://opds-spec.org/image',
-        'http://opds-spec.org/cover',
+        'http://opds-spec.org/cover', // ManyBooks legacy, not in spec
+        'x-stanza-cover-image', // Lexcycle Stanza legacy
     ],
     THUMBNAIL: [
         'http://opds-spec.org/image/thumbnail',
-        'http://opds-spec.org/thumbnail',
+        'http://opds-spec.org/thumbnail', // ManyBooks legacy, not in spec
+        'x-stanza-cover-image-thumbnail', // Lexcycle Stanza legacy
     ],
     STREAM: 'http://vaemendis.net/opds-pse/stream',
 }