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.

test-sites/demo-site/archive/mock-api/content.json

@@ -0,0 +1,88 @@
+{
+  "site": {
+    "id": "acme-consulting",
+    "domain": "acmeconsulting.example.com",
+    "owner": "client@acmeconsulting.com",
+    "created": "2024-01-15T10:30:00Z",
+    "updated": "2024-01-29T14:22:00Z"
+  },
+  "content": {
+    "nav-logo": {
+      "type": "simple",
+      "value": "Acme Consulting",
+      "updated": "2024-01-15T10:30:00Z",
+      "updatedBy": "client@acmeconsulting.com"
+    },
+    "hero-content": {
+      "type": "rich",
+      "value": "# Transform Your Business with Expert Consulting\n\nWe help small businesses grow through strategic planning, process optimization, and digital transformation. Our team brings 15+ years of experience to drive your success.\n\n[Get Started Today](contact.html)",
+      "updated": "2024-01-20T09:15:00Z",
+      "updatedBy": "client@acmeconsulting.com"
+    },
+    "services-title": {
+      "type": "rich",
+      "value": "## Our Services\n\nComprehensive solutions tailored to your business needs",
+      "updated": "2024-01-15T10:30:00Z",
+      "updatedBy": "client@acmeconsulting.com"
+    },
+    "service-strategy": {
+      "type": "rich",
+      "value": "### Strategic Planning\n\nDevelop clear roadmaps and actionable strategies that align with your business goals and drive sustainable growth.",
+      "updated": "2024-01-15T10:30:00Z",
+      "updatedBy": "client@acmeconsulting.com"
+    },
+    "service-operations": {
+      "type": "rich",
+      "value": "### Operations Optimization\n\nStreamline processes, reduce costs, and improve efficiency through proven methodologies and best practices.",
+      "updated": "2024-01-15T10:30:00Z",
+      "updatedBy": "client@acmeconsulting.com"
+    },
+    "service-digital": {
+      "type": "rich",
+      "value": "### Digital Transformation\n\nModernize your technology stack and digital presence to compete effectively in today's marketplace.",
+      "updated": "2024-01-15T10:30:00Z",
+      "updatedBy": "client@acmeconsulting.com"
+    },
+    "testimonial-content": {
+      "type": "rich",
+      "value": "> \"Acme Consulting transformed our operations completely. We saw a 40% increase in efficiency within 6 months of implementing their recommendations.\"\n> \n> — Sarah Johnson, CEO of TechStart Inc.",
+      "updated": "2024-01-25T16:45:00Z",
+      "updatedBy": "client@acmeconsulting.com"
+    },
+    "cta-content": {
+      "type": "rich",
+      "value": "## Ready to Transform Your Business?\n\nContact us today for a free consultation and discover how we can help you achieve your goals.\n\n[Schedule Consultation](contact.html)",
+      "updated": "2024-01-15T10:30:00Z",
+      "updatedBy": "client@acmeconsulting.com"
+    },
+    "footer-info": {
+      "type": "simple",
+      "value": "© 2024 Acme Consulting Services. All rights reserved.\n📧 info@acmeconsulting.com | 📞 (555) 123-4567",
+      "updated": "2024-01-15T10:30:00Z",
+      "updatedBy": "client@acmeconsulting.com"
+    }
+  },
+  "permissions": {
+    "client@acmeconsulting.com": {
+      "role": "editor",
+      "canEdit": ["*"],
+      "canDelete": false,
+      "canCreatePages": false
+    },
+    "developer@example.com": {
+      "role": "admin",
+      "canEdit": ["*"],
+      "canDelete": true,
+      "canCreatePages": true
+    }
+  },
+  "history": [
+    {
+      "contentId": "testimonial-content",
+      "timestamp": "2024-01-25T16:45:00Z",
+      "user": "client@acmeconsulting.com",
+      "action": "update",
+      "previousValue": "> \"Working with Acme Consulting was a game-changer for our business.\"\n> \n> — Sarah Johnson, CEO of TechStart Inc."
+    }
+  ]
+}