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/queries/versions.sql

@@ -1,22 +1,22 @@
 -- name: CreateContentVersion :exec
-INSERT INTO content_versions (content_id, site_id, html_content, original_template, type, created_by)
-VALUES (sqlc.arg(content_id), sqlc.arg(site_id), sqlc.arg(html_content), sqlc.arg(original_template), sqlc.arg(type), sqlc.arg(created_by));
+INSERT INTO content_versions (content_id, site_id, html_content, original_template, created_by)
+VALUES (sqlc.arg(content_id), sqlc.arg(site_id), sqlc.arg(html_content), sqlc.arg(original_template), sqlc.arg(created_by));
 
 -- 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 = sqlc.arg(content_id) AND site_id = sqlc.arg(site_id)
 ORDER BY created_at DESC
 LIMIT sqlc.arg(limit_count);
 
 -- 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 = sqlc.arg(version_id);
 
 -- 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