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/setup.sql

@@ -2,8 +2,9 @@
 CREATE TABLE IF NOT EXISTS content (
     id TEXT NOT NULL,
     site_id TEXT NOT NULL,
-    value TEXT NOT NULL,
-    type TEXT NOT NULL CHECK (type IN ('text', 'markdown', 'link')),
+    html_content TEXT NOT NULL,
+    original_template TEXT,
+    type TEXT NOT NULL CHECK (type IN ('text', 'link')),
     created_at BIGINT DEFAULT (EXTRACT(EPOCH FROM NOW())) NOT NULL,
     updated_at BIGINT DEFAULT (EXTRACT(EPOCH FROM NOW())) NOT NULL,
     last_edited_by TEXT DEFAULT 'system' NOT NULL,
@@ -15,7 +16,8 @@ CREATE TABLE IF NOT EXISTS 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