feat: add deleteAccount xrpc
Adds social.grain.unspecced.deleteAccount procedure so clients can
satisfy App Store guideline 5.1.1(v) with a single call. Deletes every
Grain record owned by the viewer across all ten grain.* collections
(photos, exif, gallery.items, galleries, stories, favorites, comments,
follows, blocks, actor profile), then clears appview-side state the user
created (mutes, preferences, push tokens) and revokes their OAuth
session. Reports and labels are moderation records and are preserved.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>