feat: Add PostgreSQL database infrastructure

- Add SQLAlchemy models for User, Translation, ApiKey, UsageLog, PaymentHistory
- Add database connection management with PostgreSQL/SQLite support
- Add repository layer for CRUD operations
- Add Alembic migration setup with initial migration
- Update auth_service to automatically use database when DATABASE_URL is set
- Update docker-compose.yml with PostgreSQL service and Redis (non-optional)
- Add database migration script (scripts/migrate_to_db.py)
- Update .env.example with database configuration

database/__init__.py

@@ -0,0 +1,17 @@
+"""
+Database module for the Document Translation API
+Provides PostgreSQL support with async SQLAlchemy
+"""
+from database.connection import get_db, engine, SessionLocal, init_db
+from database.models import User, Subscription, Translation, ApiKey
+
+__all__ = [
+    "get_db",
+    "engine", 
+    "SessionLocal",
+    "init_db",
+    "User",
+    "Subscription", 
+    "Translation",
+    "ApiKey"
+]