feat: complete HTML-first architecture implementation (Phase 1 & 2)

Major architectural simplification removing content type complexity:

Database Schema:
- Remove 'type' field from content and content_versions tables
- Simplify to pure HTML storage with html_content + original_template
- Regenerate all sqlc models for SQLite and PostgreSQL

API Simplification:
- Remove content type routing and validation
- Eliminate type-specific handlers (text/markdown/structured)
- Unified HTML-first approach for all content operations
- Simplify CreateContent and UpdateContent to HTML-only

Backend Enhancements:
- Update enhancer to only generate data-content-id (no data-content-type)
- Improve container expansion utilities with comprehensive block/inline rules
- Add Phase 3 preparation with boundary-respecting traversal logic
- Strengthen element classification for viable children detection

Documentation:
- Update TODO.md to reflect Phase 1-3 completion status
- Add WORKING_ON.md documenting the architectural transformation
- Mark container expansion and HTML-first architecture as complete

This completes the transition to a unified HTML-first content management system
with automatic style detection and element-based behavior, eliminating the
complex multi-type system in favor of semantic HTML-driven editing.

internal/engine/types.go

@@ -35,7 +35,6 @@ type ContentResult struct {
 type ProcessedElement struct {
 	Node      *html.Node // HTML node
 	ID        string     // Generated content ID
-	Type      string     // Content type (text, link)
 	Content   string     // Injected content (if any)
 	Generated bool       // Whether ID was generated (vs existing)
 	Tag       string     // Element tag name
@@ -48,7 +47,7 @@ type ContentClient interface {
 	GetContent(siteID, contentID string) (*ContentItem, error)
 	GetBulkContent(siteID string, contentIDs []string) (map[string]ContentItem, error)
 	GetAllContent(siteID string) (map[string]ContentItem, error)
-	CreateContent(siteID, contentID, htmlContent, originalTemplate, contentType, lastEditedBy string) (*ContentItem, error)
+	CreateContent(siteID, contentID, htmlContent, originalTemplate, lastEditedBy string) (*ContentItem, error)
 }
 
 // ContentItem represents a piece of content from the database
@@ -57,7 +56,6 @@ type ContentItem struct {
 	SiteID           string `json:"site_id"`
 	HTMLContent      string `json:"html_content"`
 	OriginalTemplate string `json:"original_template"`
-	Type             string `json:"type"`
 	UpdatedAt        string `json:"updated_at"`
 	LastEditedBy     string `json:"last_edited_by,omitempty"`
 }