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/repositories/base.py

@@ -0,0 +1,75 @@
+"""Base repository with generic CRUD operations."""
+
+from typing import Generic, TypeVar, Optional, List, Any
+from uuid import UUID
+from sqlalchemy.ext.asyncio import AsyncSession
+from sqlalchemy import select, delete, update, func
+
+from src.models.base import Base
+
+ModelType = TypeVar("ModelType", bound=Base)
+
+
+class BaseRepository(Generic[ModelType]):
+    """Generic base repository with common CRUD operations."""
+
+    def __init__(self, model: type[ModelType]):
+        self.model = model
+
+    async def get(self, db: AsyncSession, id: UUID) -> Optional[ModelType]:
+        """Get a single record by ID."""
+        result = await db.execute(select(self.model).where(self.model.id == id))
+        return result.scalar_one_or_none()
+
+    async def get_multi(
+        self, db: AsyncSession, *, skip: int = 0, limit: int = 100, **filters
+    ) -> List[ModelType]:
+        """Get multiple records with optional filtering."""
+        query = select(self.model)
+
+        # Apply filters
+        for key, value in filters.items():
+            if hasattr(self.model, key) and value is not None:
+                query = query.where(getattr(self.model, key) == value)
+
+        query = query.offset(skip).limit(limit)
+        result = await db.execute(query)
+        return result.scalars().all()
+
+    async def count(self, db: AsyncSession, **filters) -> int:
+        """Count records with optional filtering."""
+        query = select(func.count(self.model.id))
+
+        for key, value in filters.items():
+            if hasattr(self.model, key) and value is not None:
+                query = query.where(getattr(self.model, key) == value)
+
+        result = await db.execute(query)
+        return result.scalar()
+
+    async def create(self, db: AsyncSession, *, obj_in: dict) -> ModelType:
+        """Create a new record."""
+        db_obj = self.model(**obj_in)
+        db.add(db_obj)
+        await db.commit()
+        await db.refresh(db_obj)
+        return db_obj
+
+    async def update(
+        self, db: AsyncSession, *, db_obj: ModelType, obj_in: dict
+    ) -> ModelType:
+        """Update a record."""
+        for field, value in obj_in.items():
+            if hasattr(db_obj, field) and value is not None:
+                setattr(db_obj, field, value)
+
+        db.add(db_obj)
+        await db.commit()
+        await db.refresh(db_obj)
+        return db_obj
+
+    async def delete(self, db: AsyncSession, *, id: UUID) -> bool:
+        """Delete a record by ID."""
+        result = await db.execute(delete(self.model).where(self.model.id == id))
+        await db.commit()
+        return result.rowcount > 0