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>

tests/Entity/CharacterTest.php

@@ -42,6 +42,20 @@ class CharacterTest extends TestCase
         self::assertSame(1, $c->skillModifier('Perception'));
     }
 
+    public function testExpertiseDoublesProficiencyBonus(): void
+    {
+        $c = $this->darragh()->setSkillExpertise(['Stealth']);
+
+        // Dex 17 -> +3, expertise (2 x +2 prof) -> +7
+        self::assertSame(7, $c->skillModifier('Stealth'));
+        // Sleight of Hand stays plain proficient -> +5
+        self::assertSame(5, $c->skillModifier('Sleight of Hand'));
+
+        // Expertise alone implies proficiency
+        $c->setSkillProficiencies([])->setSkillExpertise(['Perception']);
+        self::assertSame(5, $c->skillModifier('Perception')); // Wis +1, 2 x +2
+    }
+
     public function testSaveModifierAddsProficiencyOnlyWhenProficient(): void
     {
         $c = $this->darragh();