Generate readable usernames for local actors

[2chanhaeng]

• Add @faker-js/faker to @drfed/graphql; derive usernames from
  faker.internet.username(), normalized to [a-z0-9_] for
  actors_username_check and common handle rules.
• Retry on the (username, instance_id) unique conflict.
• Build profileUrl from the username.
• Update actor.test.ts.

Tracks the FIXME in generateActor() in packages/graphql/src/actor.ts.

AI disclosure

The user asked Claude Code (claude-fable-5-1) to survey every FIXME comment in the codebase and draft an issue for each. The user read and edited the drafts, then directed Claude Code to file them through the GitHub CLI.