feat(ST-vi5iff4s): SoloTask-interface + Prisma-queries uitbreiden met PBI-velden

pbi_code, pbi_title, pbi_description (nullable) toegevoegd aan SoloTask-interface.
Desktop en mobile solo-page: story.pbi select + mapping via ?. en ?? null.
Test-fixtures bijgewerkt (3 bestanden). 72 testfiles groen, tsc + build slagen.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

__tests__/stores/solo-store-realtime.test.ts

@@ -17,6 +17,9 @@ const baseTask = (id: string, overrides: Partial<SoloTask> = {}): SoloTask => ({
   story_code: 'ST-100',
   story_title: 'Original Story',
   task_code: 'ST-100.1',
+  pbi_code: null,
+  pbi_title: null,
+  pbi_description: null,
   ...overrides,
 })