feat(lexicon): make moderationType extensible in community.profile

- Change moderationType from closed enum to knownValues
- Add to required fields (critical before alpha - can't add required later)
- Add default value "moderator" for alpha simplicity
- Add maxLength constraint per atProto style guide

This enables future moderation types without schema migration:
- "sortition" - Community tribunal (Beta Phase 1)
- "instance-labeler" - Instance moderation service
- "third-party-labeler" - External moderation DID

Per atProto style guide: enum sets cannot be extended without breaking
schema evolution. knownValues provides flexible alternative.

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