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/db/queries/content.sql

@@ -0,0 +1,39 @@
+-- name: GetContent :one
+SELECT id, site_id, value, type, created_at, updated_at, last_edited_by
+FROM content 
+WHERE id = sqlc.arg(id) AND site_id = sqlc.arg(site_id);
+
+-- name: GetAllContent :many
+SELECT id, site_id, value, type, created_at, updated_at, last_edited_by
+FROM content 
+WHERE site_id = sqlc.arg(site_id)
+ORDER BY updated_at DESC;
+
+-- name: GetBulkContent :many
+SELECT id, site_id, value, type, created_at, updated_at, last_edited_by
+FROM content 
+WHERE site_id = sqlc.arg(site_id) AND id IN (sqlc.slice('ids'));
+
+-- name: CreateContent :one
+INSERT INTO content (id, site_id, value, type, last_edited_by)
+VALUES (sqlc.arg(id), sqlc.arg(site_id), sqlc.arg(value), sqlc.arg(type), sqlc.arg(last_edited_by))
+RETURNING id, site_id, value, type, created_at, updated_at, last_edited_by;
+
+-- name: UpdateContent :one
+UPDATE content 
+SET value = sqlc.arg(value), type = sqlc.arg(type), last_edited_by = sqlc.arg(last_edited_by)
+WHERE id = sqlc.arg(id) AND site_id = sqlc.arg(site_id)
+RETURNING id, site_id, value, type, created_at, updated_at, last_edited_by;
+
+-- name: UpsertContent :one
+INSERT INTO content (id, site_id, value, type, last_edited_by)
+VALUES (sqlc.arg(id), sqlc.arg(site_id), sqlc.arg(value), sqlc.arg(type), sqlc.arg(last_edited_by))
+ON CONFLICT(id, site_id) DO UPDATE SET
+    value = EXCLUDED.value,
+    type = EXCLUDED.type,
+    last_edited_by = EXCLUDED.last_edited_by
+RETURNING id, site_id, value, type, created_at, updated_at, last_edited_by;
+
+-- name: DeleteContent :exec
+DELETE FROM content 
+WHERE id = sqlc.arg(id) AND site_id = sqlc.arg(site_id);