feat: Complete HTML-first architecture implementation with API integration

- Replace value field with html_content for direct HTML storage
- Add original_template field for style detection preservation
- Remove all markdown processing from injector (delete markdown.go)
- Fix critical content extraction/injection bugs in engine
- Add missing UpdateContent PUT handler for content persistence
- Fix API client field names and add updateContent() method
- Resolve content type validation (only allow text/link types)
- Add UUID-based ID generation to prevent collisions
- Complete first-pass processing workflow for unprocessed elements
- Verify end-to-end: Enhancement → Database → API → Editor → Persistence

All 37 files updated for HTML-first content management system.
Phase 3a implementation complete and production ready.

internal/db/postgresql/schema.sql

@@ -3,7 +3,8 @@
 CREATE TABLE content (
     id TEXT NOT NULL,
     site_id TEXT NOT NULL,
-    value TEXT NOT NULL,
+    html_content TEXT NOT NULL,
+    original_template TEXT,
     type TEXT NOT NULL,
     created_at BIGINT DEFAULT EXTRACT(EPOCH FROM NOW()) NOT NULL,
     updated_at BIGINT DEFAULT EXTRACT(EPOCH FROM NOW()) NOT NULL,
@@ -16,7 +17,8 @@ CREATE TABLE content_versions (
     version_id SERIAL PRIMARY KEY,
     content_id TEXT NOT NULL,
     site_id TEXT NOT NULL,
-    value TEXT NOT NULL,
+    html_content TEXT NOT NULL,
+    original_template TEXT,
     type TEXT NOT NULL,
     created_at BIGINT DEFAULT EXTRACT(EPOCH FROM NOW()) NOT NULL,
     created_by TEXT DEFAULT 'system' NOT NULL