feat(backend): implement database layer with models, schemas and repositories

Complete backend core implementation (BE-001 to BE-005):

BE-001: Database Connection & Session Management
- Create src/core/database.py with async SQLAlchemy 2.0
- Configure engine with pool_size=20
- Implement get_db() FastAPI dependency

BE-002: SQLAlchemy Models (5 models)
- Base model with TimestampMixin
- Scenario: status enum, relationships, cost tracking
- ScenarioLog: message hash, PII detection, metrics
- ScenarioMetric: time-series with extra_data (JSONB)
- AwsPricing: service pricing with region support
- Report: format enum, file tracking, extra_data

BE-003: Pydantic Schemas
- Scenario: Create, Update, Response, List schemas
- Log: Ingest, Response schemas
- Metric: Summary, CostBreakdown, MetricsResponse
- Common: PaginatedResponse generic type

BE-004: Base Repository Pattern
- Generic BaseRepository[T] with CRUD operations
- Methods: get, get_multi, count, create, update, delete
- Dynamic filter support

BE-005: Scenario Repository
- Extends BaseRepository[Scenario]
- Specific methods: get_by_name, list_by_status, list_by_region
- Business methods: update_status, increment_total_requests, update_total_cost
- ScenarioStatus enum
- Singleton instance: scenario_repository

All models, schemas and repositories tested and working.

Tasks: BE-001, BE-002, BE-003, BE-004, BE-005 complete

src/core/database.py

@@ -0,0 +1,41 @@
+"""Database configuration and session management."""
+
+import os
+from sqlalchemy.ext.asyncio import create_async_engine, AsyncSession, async_sessionmaker
+from sqlalchemy.orm import declarative_base
+
+# URL dal environment o default per dev
+DATABASE_URL = os.getenv(
+    "DATABASE_URL", "postgresql+asyncpg://app:changeme@localhost:5432/mockupaws"
+)
+
+# Engine async
+engine = create_async_engine(
+    DATABASE_URL,
+    echo=False,  # Set to True for debug SQL
+    future=True,
+    pool_size=20,
+    max_overflow=0,
+)
+
+# Session factory
+AsyncSessionLocal = async_sessionmaker(
+    engine,
+    class_=AsyncSession,
+    expire_on_commit=False,
+    autocommit=False,
+    autoflush=False,
+)
+
+# Base per i modelli
+Base = declarative_base()
+
+
+# Dependency per FastAPI
+async def get_db() -> AsyncSession:
+    """Dependency that provides a database session."""
+    async with AsyncSessionLocal() as session:
+        try:
+            yield session
+        finally:
+            await session.close()