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/postgresql/versions.sql.go

@@ -11,8 +11,8 @@ import (
 )
 
 const createContentVersion = `-- name: CreateContentVersion :exec
-INSERT INTO content_versions (content_id, site_id, html_content, original_template, type, created_by)
-VALUES ($1, $2, $3, $4, $5, $6)
+INSERT INTO content_versions (content_id, site_id, html_content, original_template, created_by)
+VALUES ($1, $2, $3, $4, $5)
 `
 
 type CreateContentVersionParams struct {
@@ -20,7 +20,6 @@ type CreateContentVersionParams struct {
 	SiteID           string         `json:"site_id"`
 	HtmlContent      string         `json:"html_content"`
 	OriginalTemplate sql.NullString `json:"original_template"`
-	Type             string         `json:"type"`
 	CreatedBy        string         `json:"created_by"`
 }
 
@@ -30,7 +29,6 @@ func (q *Queries) CreateContentVersion(ctx context.Context, arg CreateContentVer
 		arg.SiteID,
 		arg.HtmlContent,
 		arg.OriginalTemplate,
-		arg.Type,
 		arg.CreatedBy,
 	)
 	return err
@@ -53,7 +51,7 @@ func (q *Queries) DeleteOldVersions(ctx context.Context, arg DeleteOldVersionsPa
 
 const getAllVersionsForSite = `-- name: GetAllVersionsForSite :many
 SELECT 
-    cv.version_id, cv.content_id, cv.site_id, cv.html_content, cv.original_template, cv.type, cv.created_at, cv.created_by,
+    cv.version_id, cv.content_id, cv.site_id, cv.html_content, cv.original_template, cv.created_at, cv.created_by,
     c.html_content as current_html_content
 FROM content_versions cv
 LEFT JOIN content c ON cv.content_id = c.id AND cv.site_id = c.site_id
@@ -73,7 +71,6 @@ type GetAllVersionsForSiteRow 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"`
 	CurrentHtmlContent sql.NullString `json:"current_html_content"`
@@ -94,7 +91,6 @@ func (q *Queries) GetAllVersionsForSite(ctx context.Context, arg GetAllVersionsF
 			&i.SiteID,
 			&i.HtmlContent,
 			&i.OriginalTemplate,
-			&i.Type,
 			&i.CreatedAt,
 			&i.CreatedBy,
 			&i.CurrentHtmlContent,
@@ -113,7 +109,7 @@ func (q *Queries) GetAllVersionsForSite(ctx context.Context, arg GetAllVersionsF
 }
 
 const getContentVersion = `-- name: GetContentVersion :one
-SELECT version_id, content_id, site_id, html_content, original_template, type, created_at, created_by
+SELECT version_id, content_id, site_id, html_content, original_template, created_at, created_by
 FROM content_versions
 WHERE version_id = $1
 `
@@ -127,7 +123,6 @@ func (q *Queries) GetContentVersion(ctx context.Context, versionID int32) (Conte
 		&i.SiteID,
 		&i.HtmlContent,
 		&i.OriginalTemplate,
-		&i.Type,
 		&i.CreatedAt,
 		&i.CreatedBy,
 	)
@@ -135,7 +130,7 @@ func (q *Queries) GetContentVersion(ctx context.Context, versionID int32) (Conte
 }
 
 const getContentVersionHistory = `-- name: GetContentVersionHistory :many
-SELECT version_id, content_id, site_id, html_content, original_template, type, created_at, created_by
+SELECT version_id, content_id, site_id, html_content, original_template, created_at, created_by
 FROM content_versions
 WHERE content_id = $1 AND site_id = $2
 ORDER BY created_at DESC
@@ -163,7 +158,6 @@ func (q *Queries) GetContentVersionHistory(ctx context.Context, arg GetContentVe
 			&i.SiteID,
 			&i.HtmlContent,
 			&i.OriginalTemplate,
-			&i.Type,
 			&i.CreatedAt,
 			&i.CreatedBy,
 		); err != nil {