refactor: consolidate database structure and move injector to engine

Database Structure Cleanup:
- Move all SQL files from ./db/ to ./internal/db/
- Update sqlc.yaml to use new paths (preserving schema+setup.sql hack)
- Consolidate database-related code in single directory
- Remove empty ./db/ directory

Injector Migration:
- Move injector.go from content package to engine package
- Update ContentClient interface to return map instead of slice for GetBulkContent
- Update database client implementation to match interface
- Remove injector dependency from enhancer (stub implementation)

Demo-Site Consolidation:
- Move demo-site to test-sites/demo-site for better organization
- Update build scripts to use new demo-site location
- Maintain all functionality while improving project structure

This continues the unified architecture consolidation by moving core content
processing logic to the engine and organizing related files properly.

scripts/dev.js

@@ -39,8 +39,8 @@ const commands = {
       
       // Check files exist
       const requiredFiles = [
-        'demo-site/index.html',
-        'demo-site/about.html', 
+        'test-sites/demo-site/index.html',
+        'test-sites/demo-site/about.html', 
         'lib/dist/insertr.js',
         'lib/dist/insertr.min.js',
         'cmd/serve.go',
@@ -77,8 +77,8 @@ const commands = {
         console.log('\n📊 Project stats:');
         
         // Count editable elements
-        const indexContent = fs.readFileSync('demo-site/index.html', 'utf8');
-        const aboutContent = fs.readFileSync('demo-site/about.html', 'utf8');
+        const indexContent = fs.readFileSync('test-sites/demo-site/index.html', 'utf8');
+        const aboutContent = fs.readFileSync('test-sites/demo-site/about.html', 'utf8');
         const insertrMatches = (indexContent + aboutContent).match(/class="insertr"/g) || [];
         console.log(`   📝 Editable elements: ${insertrMatches.length}`);