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/database_client.go

@@ -46,7 +46,6 @@ func (c *DatabaseClient) GetContent(siteID, contentID string) (*ContentItem, err
 			SiteID:           content.SiteID,
 			HTMLContent:      content.HtmlContent,
 			OriginalTemplate: getStringFromNullString(content.OriginalTemplate),
-			Type:             content.Type,
 			LastEditedBy:     content.LastEditedBy,
 		}, nil
 
@@ -63,7 +62,6 @@ func (c *DatabaseClient) GetContent(siteID, contentID string) (*ContentItem, err
 			SiteID:           content.SiteID,
 			HTMLContent:      content.HtmlContent,
 			OriginalTemplate: getStringFromNullString(content.OriginalTemplate),
-			Type:             content.Type,
 			LastEditedBy:     content.LastEditedBy,
 		}, nil
 
@@ -91,7 +89,6 @@ func (c *DatabaseClient) GetBulkContent(siteID string, contentIDs []string) (map
 				SiteID:           content.SiteID,
 				HTMLContent:      content.HtmlContent,
 				OriginalTemplate: getStringFromNullString(content.OriginalTemplate),
-				Type:             content.Type,
 				LastEditedBy:     content.LastEditedBy,
 			}
 		}
@@ -113,7 +110,6 @@ func (c *DatabaseClient) GetBulkContent(siteID string, contentIDs []string) (map
 				SiteID:           content.SiteID,
 				HTMLContent:      content.HtmlContent,
 				OriginalTemplate: getStringFromNullString(content.OriginalTemplate),
-				Type:             content.Type,
 				LastEditedBy:     content.LastEditedBy,
 			}
 		}
@@ -140,7 +136,6 @@ func (c *DatabaseClient) GetAllContent(siteID string) (map[string]ContentItem, e
 				SiteID:           content.SiteID,
 				HTMLContent:      content.HtmlContent,
 				OriginalTemplate: getStringFromNullString(content.OriginalTemplate),
-				Type:             content.Type,
 				LastEditedBy:     content.LastEditedBy,
 			}
 		}
@@ -159,7 +154,6 @@ func (c *DatabaseClient) GetAllContent(siteID string) (map[string]ContentItem, e
 				SiteID:           content.SiteID,
 				HTMLContent:      content.HtmlContent,
 				OriginalTemplate: getStringFromNullString(content.OriginalTemplate),
-				Type:             content.Type,
 				LastEditedBy:     content.LastEditedBy,
 			}
 		}
@@ -171,7 +165,7 @@ func (c *DatabaseClient) GetAllContent(siteID string) (map[string]ContentItem, e
 }
 
 // CreateContent creates a new content item
-func (c *DatabaseClient) CreateContent(siteID, contentID, htmlContent, originalTemplate, contentType, lastEditedBy string) (*ContentItem, error) {
+func (c *DatabaseClient) CreateContent(siteID, contentID, htmlContent, originalTemplate, lastEditedBy string) (*ContentItem, error) {
 	switch c.database.GetDBType() {
 	case "sqlite3":
 		content, err := c.database.GetSQLiteQueries().CreateContent(context.Background(), sqlite.CreateContentParams{
@@ -179,7 +173,6 @@ func (c *DatabaseClient) CreateContent(siteID, contentID, htmlContent, originalT
 			SiteID:           siteID,
 			HtmlContent:      htmlContent,
 			OriginalTemplate: toNullString(originalTemplate),
-			Type:             contentType,
 			LastEditedBy:     lastEditedBy,
 		})
 		if err != nil {
@@ -190,7 +183,6 @@ 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
 
@@ -200,7 +192,6 @@ func (c *DatabaseClient) CreateContent(siteID, contentID, htmlContent, originalT
 			SiteID:           siteID,
 			HtmlContent:      htmlContent,
 			OriginalTemplate: toNullString(originalTemplate),
-			Type:             contentType,
 			LastEditedBy:     lastEditedBy,
 		})
 		if err != nil {
@@ -211,7 +202,6 @@ 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
 
@@ -220,7 +210,7 @@ func (c *DatabaseClient) CreateContent(siteID, contentID, htmlContent, originalT
 	}
 }
 
-// Helper function to convert string to sql.NullString  
+// Helper function to convert string to sql.NullString
 func toNullString(s string) sql.NullString {
 	if s == "" {
 		return sql.NullString{Valid: false}