Store Weasyl and e621 links on artists so lookups and imports can match them

[sparkyfen]

Context

The artists table stores per-site links for Twitter, Bluesky, Telegram, FurAffinity, DeviantArt, Patreon, and Instagram, but not Weasyl or e621. FuzzySearch (#288) returns handles on both sites, and the #151 import sources list has both marked as coming later. Without columns for them, a lookup that matches only on Weasyl or e621 can create a new artist but can never link an existing one, so repeat pieces by the same artist fragment into duplicate rows. Twitter coverage in FuzzySearch looks per-tracked-account, which makes this the common case rather than the edge case.

Split out of #288 by operator decision on 2026-09-03.

Scope

• Migration adding nullable weasyl_url and e621_url to artists.
• Two fields on the artist form and the New Artist dialog, with icons and URL placeholders matching the existing socials.
• Two icons on the public artist card and the /about socials rows.
• Registry sync: decide whether the shared registry carries the same two fields; if not, they stay local-only and artist-sync must not blank them.
• FuzzySearch artist lookup: suggest artist/source/date/rating on upload, edit, and import review #288 resolution and prefill widen to four sites once this lands; the "Sona can't link Weasyl or e621 handles" callout goes away.

Rollout

Migration rides the existing migration mechanism on every fork; no per-fork manual step.

[linear-code]

SONA-219