Reorganize Docker-related files into docker/ directory

Moved Docker infrastructure files to dedicated docker/ folder:
- config/nginx/default.conf → docker/config/nginx/default.conf
- init.sql → docker/init.sql
- start.sh → docker/start.sh (currently unused)

Updated:
- docker-compose.yml: Adjusted volume paths
- README.md: Updated project structure documentation

Benefits:
- Clear separation between infrastructure (docker/) and application (src/)
- Better project organization
- Easier to understand for new developers

Docker Compose and Dockerfile remain in root for convenience.
All services tested and working correctly.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

docker/start.sh

@@ -0,0 +1,7 @@
+#!/bin/bash
+
+# Start PHP-FPM
+php-fpm -D
+
+# Start Nginx in foreground
+nginx -g 'daemon off;'