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/setup.sql

@@ -4,7 +4,6 @@ CREATE TABLE IF NOT EXISTS content (
     site_id TEXT NOT NULL,
     html_content TEXT NOT NULL,
     original_template TEXT,
-    type TEXT NOT NULL CHECK (type IN ('text', 'link')),
     created_at INTEGER DEFAULT (strftime('%s', 'now')) NOT NULL,
     updated_at INTEGER DEFAULT (strftime('%s', 'now')) NOT NULL,
     last_edited_by TEXT DEFAULT 'system' NOT NULL,
@@ -18,7 +17,6 @@ CREATE TABLE IF NOT EXISTS content_versions (
     site_id TEXT NOT NULL,
     html_content TEXT NOT NULL,
     original_template TEXT,
-    type TEXT NOT NULL,
     created_at INTEGER DEFAULT (strftime('%s', 'now')) NOT NULL,
     created_by TEXT DEFAULT 'system' NOT NULL
 );