feat: add use cases marketing product with AI-generated content

Transform qualifying scenarios into marketing-ready use cases with:
- AI-generated titles, descriptions, ROI estimates, business value
- Persona/industry/category taxonomy for targeting
- Browseable feed with filtering and ranking
- SEO-optimized case study pages
- Daily cron job for generation and ranking

Database:
- Add Persona, Industry, Category lookup tables
- Add UseCase model with marketing content fields
- Add junction tables for personas/industries/categories
- Add SocialProof model for cached metrics

API:
- GET /api/use-cases - Global directory with filtering
- GET /api/use-cases/[id] - Individual use case details
- GET /api/public/users/[username]/collections/[slug]/use-cases
- POST /api/cron/use-cases - Nightly generation job

Frontend:
- /use-cases - Global feed with persona dropdown
- /use-cases/[slug] - SEO case study page
- /[username]/collections/[slug]/use-cases - Collection feed
- UseCasesFeed component - Sortable table component
- UseCaseCaseStudy component - Full case study layout

.vscode/settings.json

@@ -2,8 +2,7 @@
   "editor.formatOnSave": true,
   "editor.defaultFormatter": "biomejs.biome",
   "editor.codeActionsOnSave": {
-    "quickfix.biome": "explicit",
-    "source.organizeImports.biome": "explicit"
+    "source.fixAll.biome": "explicit"
   },
   "typescript.tsdk": "node_modules/typescript/lib",
   "typescript.enablePromptUseWorkspaceTsdk": true