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.

justfile

@@ -54,7 +54,7 @@ dev: build-lib build
     echo ""
     
     # Start demo site with prefixed output (this will block) - use local installation
-    cd {{justfile_directory()}} && npx --prefer-offline live-server demo-site --port=3000 --host=localhost --open=/index.html 2>&1 | sed 's/^/🌐 [DEMO] /' &
+    cd {{justfile_directory()}} && npx --prefer-offline live-server test-sites/demo-site --port=3000 --host=localhost --open=/index.html 2>&1 | sed 's/^/🌐 [DEMO] /' &
     DEMO_PID=$!
     
     # Wait for both processes
@@ -70,7 +70,7 @@ dev-about: build-lib build
     INSERTR_DATABASE_PATH=./dev.db ./insertr serve --dev-mode &
     SERVER_PID=$!
     sleep 3
-    npx --prefer-offline live-server demo-site --port=3000 --host=localhost --open=/about.html
+    npx --prefer-offline live-server test-sites/demo-site --port=3000 --host=localhost --open=/about.html
     kill $SERVER_PID 2>/dev/null || true
 
 # Check project status and validate setup
@@ -208,7 +208,7 @@ help:
 
 
 # Enhance demo site (build-time content injection)
-enhance input="demo-site" output="dist":
+enhance input="test-sites/demo-site" output="dist":
     ./insertr enhance {{input}} --output {{output}} --mock
 
 # === Content API Server Commands ===
@@ -267,7 +267,7 @@ status:
     @echo "\n🔧 Unified binary:"
     @ls -la insertr main.go cmd/ internal/ 2>/dev/null || echo "   Missing unified binary components"
     @echo "\n🌐 Demo site:"
-    @ls -la demo-site/index.html demo-site/about.html 2>/dev/null || echo "   Missing demo files"
+    @ls -la test-sites/demo-site/index.html test-sites/demo-site/about.html 2>/dev/null || echo "   Missing demo files"
     @echo ""
     @echo "🚀 Development Commands:"
     @echo "   just dev             - Full-stack development (recommended)"