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.

internal/db/queries/versions.sql

@@ -1,23 +1,23 @@
 -- name: CreateContentVersion :exec
-INSERT INTO content_versions (content_id, site_id, value, type, created_by)
-VALUES (sqlc.arg(content_id), sqlc.arg(site_id), sqlc.arg(value), sqlc.arg(type), sqlc.arg(created_by));
+INSERT INTO content_versions (content_id, site_id, html_content, original_template, type, created_by)
+VALUES (sqlc.arg(content_id), sqlc.arg(site_id), sqlc.arg(html_content), sqlc.arg(original_template), sqlc.arg(type), sqlc.arg(created_by));
 
 -- name: GetContentVersionHistory :many
-SELECT version_id, content_id, site_id, value, type, created_at, created_by
+SELECT version_id, content_id, site_id, html_content, original_template, type, created_at, created_by
 FROM content_versions
 WHERE content_id = sqlc.arg(content_id) AND site_id = sqlc.arg(site_id)
 ORDER BY created_at DESC
 LIMIT sqlc.arg(limit_count);
 
 -- name: GetContentVersion :one
-SELECT version_id, content_id, site_id, value, type, created_at, created_by
+SELECT version_id, content_id, site_id, html_content, original_template, type, created_at, created_by
 FROM content_versions
 WHERE version_id = sqlc.arg(version_id);
 
 -- name: GetAllVersionsForSite :many
 SELECT 
-    cv.version_id, cv.content_id, cv.site_id, cv.value, cv.type, cv.created_at, cv.created_by,
-    c.value as current_value
+    cv.version_id, cv.content_id, cv.site_id, cv.html_content, cv.original_template, cv.type, cv.created_at, cv.created_by,
+    c.html_content as current_html_content
 FROM content_versions cv
 LEFT JOIN content c ON cv.content_id = c.id AND cv.site_id = c.site_id
 WHERE cv.site_id = sqlc.arg(site_id)