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/api/models.go

@@ -8,7 +8,6 @@ type ContentItem struct {
 	SiteID           string    `json:"site_id"`
 	HTMLContent      string    `json:"html_content"`
 	OriginalTemplate string    `json:"original_template"`
-	Type             string    `json:"type"`
 	CreatedAt        time.Time `json:"created_at"`
 	UpdatedAt        time.Time `json:"updated_at"`
 	LastEditedBy     string    `json:"last_edited_by"`
@@ -20,7 +19,6 @@ type ContentVersion struct {
 	SiteID           string    `json:"site_id"`
 	HTMLContent      string    `json:"html_content"`
 	OriginalTemplate string    `json:"original_template"`
-	Type             string    `json:"type"`
 	CreatedAt        time.Time `json:"created_at"`
 	CreatedBy        string    `json:"created_by"`
 }
@@ -48,7 +46,6 @@ type CreateContentRequest struct {
 	FilePath         string `json:"file_path"`            // File path for consistent ID generation
 	HTMLContent      string `json:"html_content"`         // HTML content value
 	OriginalTemplate string `json:"original_template"`    // Original template markup
-	Type             string `json:"type"`                 // Content type
 	SiteID           string `json:"site_id,omitempty"`    // Site identifier
 	CreatedBy        string `json:"created_by,omitempty"` // User who created the content
 }