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/content/client.go

@@ -166,7 +166,7 @@ func (c *HTTPClient) GetAllContent(siteID string) (map[string]engine.ContentItem
 }
 
 // CreateContent creates a new content item via HTTP API
-func (c *HTTPClient) CreateContent(siteID, contentID, htmlContent, originalTemplate, contentType, lastEditedBy string) (*engine.ContentItem, error) {
+func (c *HTTPClient) CreateContent(siteID, contentID, htmlContent, originalTemplate, lastEditedBy string) (*engine.ContentItem, error) {
 	// For now, HTTPClient CreateContent is not implemented for enhancer use
 	// This would typically be used in API-driven enhancement scenarios
 	return nil, fmt.Errorf("CreateContent not implemented for HTTPClient - use DatabaseClient for enhancement")