Characters: seed the current party sheets, add skill expertise

The party sheets in assets/characters/ changed: Darrah_Oakwanderer.pdf is
gone, replaced by Darragh Fermwyr (same halfling ranger, renamed and
updated), Owayn Fermwyr (stout halfling fighter, BJ) and Dunnchadh
(ghostwise halfling druid, Circle of the Blighted, Rick).

- app:character:seed now carries those three sheets, read from the PDFs'
  form fields. Darragh's portrait is renamed to match his new name.
- New skill expertise field (double proficiency bonus), since Darragh's
  Deft Explorer gives him +7 Stealth and the sheet had nowhere to show
  it. Expertise implies proficiency; the sheet marks it with two dots,
  the edit form gets a checkbox list next to skill proficiencies.
- Migration adds skill_expertise, backfilling existing rows with [] so
  the NOT NULL JSON column works on tables that already have characters.

Tests: expertise math on the entity, and a catalog test that every seed
sheet hydrates into a Character that passes validation.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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