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/versions.sql

@@ -0,0 +1,29 @@
+-- name: CreateContentVersion :exec
+INSERT INTO content_versions (content_id, site_id, value, type, created_by)
+VALUES (sqlc.arg(content_id), sqlc.arg(site_id), sqlc.arg(value), sqlc.arg(type), sqlc.arg(created_by));
+
+-- name: GetContentVersionHistory :many
+SELECT version_id, content_id, site_id, value, type, created_at, created_by
+FROM content_versions
+WHERE content_id = sqlc.arg(content_id) AND site_id = sqlc.arg(site_id)
+ORDER BY created_at DESC
+LIMIT sqlc.arg(limit_count);
+
+-- name: GetContentVersion :one
+SELECT version_id, content_id, site_id, value, type, created_at, created_by
+FROM content_versions
+WHERE version_id = sqlc.arg(version_id);
+
+-- name: GetAllVersionsForSite :many
+SELECT 
+    cv.version_id, cv.content_id, cv.site_id, cv.value, cv.type, cv.created_at, cv.created_by,
+    c.value as current_value
+FROM content_versions cv
+LEFT JOIN content c ON cv.content_id = c.id AND cv.site_id = c.site_id
+WHERE cv.site_id = sqlc.arg(site_id)
+ORDER BY cv.created_at DESC
+LIMIT sqlc.arg(limit_count);
+
+-- name: DeleteOldVersions :exec
+DELETE FROM content_versions 
+WHERE created_at < sqlc.arg(created_before) AND site_id = sqlc.arg(site_id);