opds2: emit rel=alternate links for off-site identifier URLs

[ajslater]

Summary

Per beville's follow-up on #700: a comic's identifier URLs (ComicVine, Metron, etc.) are off-site representations of the same publication and belong in publication.links as rel=alternate. Mirrors the readino feed shape beville cited:

```json
"links": [
{"rel": "self", "type": "application/divina+json", "href": "..."},
{"rel": "alternate", "type": "text/html", "href": "https://comicvine.gamespot.com/...", "title": "View on ComicVine"}
]
```

Now emitted on both the per-comic manifest endpoint and on each publication in the feed.

Implementation notes

• Base view (OPDS2PublicationBaseView) runs a per-comic identifier query — used by the manifest path, which serves one book per request.
• Feed view (OPDS2PublicationsView) overrides with a batched query keyed by comic pk, populated in get_publications before the per-publication loop. Mirrors the existing credits / subjects batching, so a full feed page stays at one identifier query regardless of book count.
• URLField(default="") means we filter .exclude(url="") rather than __isnull.
• Title is "View on <source>" when the identifier has a source, "View externally" otherwise.

Test plan

• make fix, ruff, basedpyright, complexity, make test-python all clean
• Hit /opds/v2.0/c/<pk>/1 on a comic with ComicVine/Metron identifiers and confirm links contains rel=alternate entries
• Hit /opds/v2.0/<group>/<pks>/1 (a feed page) and confirm each publication's links contains its identifier alternates
• Confirm comics with no URL'd identifiers don't get spurious links

🤖 Generated with Claude Code