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/db/sqlite/models.go

@@ -13,7 +13,6 @@ type Content struct {
 	SiteID           string         `json:"site_id"`
 	HtmlContent      string         `json:"html_content"`
 	OriginalTemplate sql.NullString `json:"original_template"`
-	Type             string         `json:"type"`
 	CreatedAt        int64          `json:"created_at"`
 	UpdatedAt        int64          `json:"updated_at"`
 	LastEditedBy     string         `json:"last_edited_by"`
@@ -25,7 +24,6 @@ type ContentVersion struct {
 	SiteID           string         `json:"site_id"`
 	HtmlContent      string         `json:"html_content"`
 	OriginalTemplate sql.NullString `json:"original_template"`
-	Type             string         `json:"type"`
 	CreatedAt        int64          `json:"created_at"`
 	CreatedBy        string         `json:"created_by"`
 }