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/postgresql/models.go

@@ -1,25 +1,31 @@
 // Code generated by sqlc. DO NOT EDIT.
 // versions:
-//   sqlc v1.29.0
+//   sqlc v1.30.0
 
 package postgresql
 
+import (
+	"database/sql"
+)
+
 type Content struct {
-	ID           string `json:"id"`
-	SiteID       string `json:"site_id"`
-	Value        string `json:"value"`
-	Type         string `json:"type"`
-	CreatedAt    int64  `json:"created_at"`
-	UpdatedAt    int64  `json:"updated_at"`
-	LastEditedBy string `json:"last_edited_by"`
+	ID               string         `json:"id"`
+	SiteID           string         `json:"site_id"`
+	HtmlContent      string         `json:"html_content"`
+	OriginalTemplate sql.NullString `json:"original_template"`
+	Type             string         `json:"type"`
+	CreatedAt        int64          `json:"created_at"`
+	UpdatedAt        int64          `json:"updated_at"`
+	LastEditedBy     string         `json:"last_edited_by"`
 }
 
 type ContentVersion struct {
-	VersionID int32  `json:"version_id"`
-	ContentID string `json:"content_id"`
-	SiteID    string `json:"site_id"`
-	Value     string `json:"value"`
-	Type      string `json:"type"`
-	CreatedAt int64  `json:"created_at"`
-	CreatedBy string `json:"created_by"`
+	VersionID        int32          `json:"version_id"`
+	ContentID        string         `json:"content_id"`
+	SiteID           string         `json:"site_id"`
+	HtmlContent      string         `json:"html_content"`
+	OriginalTemplate sql.NullString `json:"original_template"`
+	Type             string         `json:"type"`
+	CreatedAt        int64          `json:"created_at"`
+	CreatedBy        string         `json:"created_by"`
 }