Add `https://purl.archive.org/miscellany` to preloaded JSON-LD contexts

[mike182uk]

Bridgy Fed (via granary) includes https://purl.archive.org/miscellany in the @context of every activity it delivers. The URL serves the SWICG "ActivityPub Miscellaneous Terms" context and is hosted on purl.archive.org, which in our experience is unreliable: over the last 90 days we've observed failures resolving this context on 28 separate days, typically in multi-hour bursts

When purl.archive.org is unreachable, Activity.fromJsonLd() fails with jsonld.InvalidUrl ("loading remote context failed") for every Bridgy Fed originated activity, before any inbox listener runs - the same failure mode that motivated preloading Lemmy's context (#714) and http://joinmastodon.org/ns (#630). The default kvCache TTL of 5 minutes
offers no protection against outages longer than that

For context: on our deployment (Ghost's ActivityPub service), purl.archive.org outages cause these failures every few days - most recently for several hours on both Jul 19 and Jul 20 - each time producing sender retry storms and 5xx alerting noise

The context seems like a good preload candidate:

• It's a SWICG deliverable, deliberately stable and versioned, with an immutable URL for the current version
  (https://purl.archive.org/miscellany/1.0.0)
• It's small (four term definitions: Hashtag, manuallyApprovesFollowers, movedTo, sensitive) all aliasing AS2 terms
• Fedify already preloads from the same unreliable host: https://purl.archive.org/socialweb/webfinger
• Fedify's own vocab docs reference the spec (hashtag.yaml links to https://swicg.github.io/miscellany/#Hashtag)

I have a PR ready (same shape as the Lemmy change in #714: JSON asset + preloadedContexts entry). Will open it referencing this issue