feat: project scaffolding and env config loader

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.env.example

@@ -0,0 +1,12 @@
+PORT=3000
+STORAGE_DIR=./storage
+DB_HOST=127.0.0.1
+DB_USER=convert_user
+DB_PASSWORD=change_me
+DB_NAME=file_converter
+MAX_FILE_SIZE_MB=100
+RETENTION_HOURS=1
+WORKER_POLL_INTERVAL_MS=1500
+WORKER_CONCURRENCY=3
+RATE_LIMIT_MAX_JOBS=20
+RATE_LIMIT_WINDOW_MINUTES=10