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.

internal/content/enhancer.go

@@ -10,8 +10,8 @@ import (
 
 // Enhancer combines parsing and content injection using unified engine
 type Enhancer struct {
-	engine   *engine.ContentEngine
-	injector *Injector
+	engine *engine.ContentEngine
+	// injector functionality will be integrated into engine
 }
 
 // NewEnhancer creates a new HTML enhancer using unified engine
@@ -26,8 +26,7 @@ func NewEnhancer(client ContentClient, siteID string) *Enhancer {
 	}
 
 	return &Enhancer{
-		engine:   engine.NewContentEngine(engineClient),
-		injector: NewInjector(client, siteID),
+		engine: engine.NewContentEngine(engineClient),
 	}
 }