feat(T01): auth + user management + RBAC + base frontend + i18n

- Backend: FastAPI, JWT auth (HS256), bcrypt, RBAC middleware
- User CRUD (admin-only), soft-delete, pagination
- Pydantic BaseSettings config, async SQLAlchemy 2.0
- 50 backend tests, 88% coverage
- Frontend: Next.js 14 App Router, Tailwind, design tokens
- Login page, auth context, API client with auto-refresh
- i18n: next-intl, DE/EN (31 keys each)
- 6 base UI components (Button, Input, Card, Table, Modal, Toast)
- 12 frontend tests, npm build success

backend/app/database.py

@@ -0,0 +1,55 @@
+"""Async SQLAlchemy database engine, session factory, and base model."""
+
+from sqlalchemy.ext.asyncio import (
+    AsyncSession,
+    async_sessionmaker,
+    create_async_engine,
+)
+from sqlalchemy.orm import DeclarativeBase
+
+from app.config import settings
+
+
+class Base(DeclarativeBase):
+    """Declarative base for all SQLAlchemy models."""
+    pass
+
+
+engine = create_async_engine(
+    settings.DATABASE_URL,
+    echo=(settings.APP_ENV == "development"),
+    pool_pre_ping=True,
+    pool_size=10,
+    max_overflow=20,
+)
+
+async_session_factory = async_sessionmaker(
+    engine,
+    class_=AsyncSession,
+    expire_on_commit=False,
+)
+
+
+async def get_db() -> AsyncSession:
+    """FastAPI dependency that yields an async DB session."""
+    async with async_session_factory() as session:
+        try:
+            yield session
+            await session.commit()
+        except Exception:
+            await session.rollback()
+            raise
+        finally:
+            await session.close()
+
+
+async def init_db():
+    """Create all tables. Used for testing and first-run setup."""
+    async with engine.begin() as conn:
+        await conn.run_sync(Base.metadata.create_all)
+
+
+async def drop_db():
+    """Drop all tables. Used in test teardown."""
+    async with engine.begin() as conn:
+        await conn.run_sync(Base.metadata.drop_all)