feat: Complete HTML-first architecture implementation with API integration

- Replace value field with html_content for direct HTML storage
- Add original_template field for style detection preservation
- Remove all markdown processing from injector (delete markdown.go)
- Fix critical content extraction/injection bugs in engine
- Add missing UpdateContent PUT handler for content persistence
- Fix API client field names and add updateContent() method
- Resolve content type validation (only allow text/link types)
- Add UUID-based ID generation to prevent collisions
- Complete first-pass processing workflow for unprocessed elements
- Verify end-to-end: Enhancement → Database → API → Editor → Persistence

All 37 files updated for HTML-first content management system.
Phase 3a implementation complete and production ready.

go.mod

@@ -7,11 +7,11 @@ require (
 	github.com/go-chi/chi/v5 v5.2.3
 	github.com/go-chi/cors v1.2.2
 	github.com/golang-jwt/jwt/v5 v5.3.0
+	github.com/google/uuid v1.6.0
 	github.com/lib/pq v1.10.9
 	github.com/mattn/go-sqlite3 v1.14.32
 	github.com/spf13/cobra v1.8.0
 	github.com/spf13/viper v1.18.2
-	github.com/yuin/goldmark v1.7.8
 	golang.org/x/net v0.43.0
 	golang.org/x/oauth2 v0.31.0
 )