Add per-artifact public sharing

Any artifact can now be made publicly viewable via a share link, no login
required - opt-in per artifact, toggled from the feed UI next to Keep.

- Artifact.share_token: nullable, unique, high-entropy (secrets.token_urlsafe(32)).
  NULL = not shared; SQL treats each NULL as distinct so uniqueness still
  holds for the many not-shared rows.
- New public route GET /s/<token> (shared_artifact_view), completely
  unauthenticated, serving the same rendered/raw content as the owner
  route via a shared serve_artifact_file() helper.
- toggle_artifact_share: generates/clears the token: mirrors
  toggle_artifact_keep's pattern.
- Fixed a real gap this surfaced: embedded images in a shared artifact's
  content would otherwise break for an anonymous visitor, since asset_view
  was owner-only. It now also allows access when the asset's parent
  artifact is currently shared - unattached (orphaned) assets stay private
  regardless.
- serialize_artifact now reports is_shared/share_url so the API reflects
  sharing status (read-only for now; toggling stays UI-only).
- Feed UI: Share button (mirrors Keep's toggle interaction), a copyable
  share-link box that appears when shared, and a "shared" badge on the
  artifact row - distinct blue accent from Keep's green so both can be
  visible on the same row without confusion.

Verified live end-to-end (toggle on/off, public access, fake-token 404,
old-link-dies-on-unshare, embedded-asset access before/after sharing)
before writing the 12 automated tests in tests/views/test_sharing.py.
Also fixed two unrelated stale spots noticed in docs/api.md while adding
the Sharing section: the example response was missing is_kept/size_bytes,
and the raw-content table didn't list json/csv (added earlier this
session but never documented there).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

CLAUDE.md

@@ -12,6 +12,7 @@ The core product principle is: agents publish content, not files. The server own
 - Require `source` in every artifact payload — a free-text label identifying what posted it (agent-profile, script, or person name). Ownership of the resulting artifact comes from whichever user's token authenticated the POST, not from the `source` value itself.
 - `source` and `tags` together form one unified **label** system: every artifact shows all its labels next to the title, and a single `label` filter/query param matches against either.
 - Require login (Django session auth) for the whole app, including the feed and API reads. Ingest (POST/PATCH) stays per-user bearer-token authenticated, since agents don't have browser sessions.
+- Exception to the above: an artifact can be explicitly opted into public sharing (per-artifact, owner-toggled from the feed UI) via a high-entropy `share_token`, served unauthenticated at `GET /s/{share_token}`. Not exposed via the ingest API yet — UI-only. Turning sharing off immediately invalidates the old link. Images embedded in a shared artifact are also served unauthenticated while it's shared, so inline content doesn't break for a link recipient.
 - User accounts can be created from Settings → Users (superuser-only in-app page) or via Django admin. Regular users are not staff and cannot access `/admin/`. Creating a user via Settings → Users also issues their API token immediately, shown once.
 - `TagColor` (label color overrides) stays global/shared across all users.
 - Use random 8-character artifact IDs.