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/database.go

@@ -144,7 +144,6 @@ func (d *DatabaseClient) convertToContentItem(content interface{}) engine.Conten
 			SiteID:           c.SiteID,
 			HTMLContent:      c.HtmlContent,
 			OriginalTemplate: getStringFromNullString(c.OriginalTemplate),
-			Type:             c.Type,
 			UpdatedAt:        time.Unix(c.UpdatedAt, 0).Format(time.RFC3339),
 		}
 	case "postgresql":
@@ -154,7 +153,6 @@ func (d *DatabaseClient) convertToContentItem(content interface{}) engine.Conten
 			SiteID:           c.SiteID,
 			HTMLContent:      c.HtmlContent,
 			OriginalTemplate: getStringFromNullString(c.OriginalTemplate),
-			Type:             c.Type,
 			UpdatedAt:        time.Unix(c.UpdatedAt, 0).Format(time.RFC3339),
 		}
 	}
@@ -182,8 +180,8 @@ func (d *DatabaseClient) convertToContentItemList(contentList interface{}) []eng
 	return []engine.ContentItem{} // Should never happen
 }
 
-// CreateContent creates a new content item  
-func (c *DatabaseClient) CreateContent(siteID, contentID, htmlContent, originalTemplate, contentType, lastEditedBy string) (*engine.ContentItem, error) {
+// CreateContent creates a new content item
+func (c *DatabaseClient) CreateContent(siteID, contentID, htmlContent, originalTemplate, lastEditedBy string) (*engine.ContentItem, error) {
 	switch c.db.GetDBType() {
 	case "sqlite3":
 		content, err := c.db.GetSQLiteQueries().CreateContent(context.Background(), sqlite.CreateContentParams{
@@ -191,7 +189,6 @@ func (c *DatabaseClient) CreateContent(siteID, contentID, htmlContent, originalT
 			SiteID:           siteID,
 			HtmlContent:      htmlContent,
 			OriginalTemplate: toNullString(originalTemplate),
-			Type:             contentType,
 			LastEditedBy:     lastEditedBy,
 		})
 		if err != nil {
@@ -202,7 +199,6 @@ func (c *DatabaseClient) CreateContent(siteID, contentID, htmlContent, originalT
 			SiteID:           content.SiteID,
 			HTMLContent:      content.HtmlContent,
 			OriginalTemplate: getStringFromNullString(content.OriginalTemplate),
-			Type:             content.Type,
 			LastEditedBy:     content.LastEditedBy,
 		}, nil
 
@@ -212,7 +208,6 @@ func (c *DatabaseClient) CreateContent(siteID, contentID, htmlContent, originalT
 			SiteID:           siteID,
 			HtmlContent:      htmlContent,
 			OriginalTemplate: toNullString(originalTemplate),
-			Type:             contentType,
 			LastEditedBy:     lastEditedBy,
 		})
 		if err != nil {
@@ -223,7 +218,6 @@ func (c *DatabaseClient) CreateContent(siteID, contentID, htmlContent, originalT
 			SiteID:           content.SiteID,
 			HTMLContent:      content.HtmlContent,
 			OriginalTemplate: getStringFromNullString(content.OriginalTemplate),
-			Type:             content.Type,
 			LastEditedBy:     content.LastEditedBy,
 		}, nil