feat(api): implement complete API layer with services and endpoints

Complete API implementation (BE-006 to BE-010):

BE-006: API Dependencies & Configuration
- Add core/config.py with Settings and environment variables
- Add core/exceptions.py with AppException hierarchy
- Add api/deps.py with get_db() and get_running_scenario() dependencies
- Add pydantic-settings dependency

BE-007: Services Layer
- Add services/pii_detector.py: PIIDetector with email/SSN/credit card patterns
- Add services/cost_calculator.py: AWS cost calculation (SQS, Lambda, Bedrock)
- Add services/ingest_service.py: Log processing with hash, PII detection, metrics

BE-008: Scenarios API Endpoints
- POST /api/v1/scenarios - Create scenario
- GET /api/v1/scenarios - List with filters and pagination
- GET /api/v1/scenarios/{id} - Get single scenario
- PUT /api/v1/scenarios/{id} - Update scenario
- DELETE /api/v1/scenarios/{id} - Delete scenario
- POST /api/v1/scenarios/{id}/start - Start (draft->running)
- POST /api/v1/scenarios/{id}/stop - Stop (running->completed)
- POST /api/v1/scenarios/{id}/archive - Archive (completed->archived)

BE-009: Ingest API
- POST /ingest with X-Scenario-ID header validation
- Depends on get_running_scenario() for status check
- Returns LogResponse with processed metrics
- POST /flush for backward compatibility

BE-010: Metrics API
- GET /api/v1/scenarios/{id}/metrics - Full metrics endpoint
- Aggregates data from scenario_logs
- Calculates costs using CostCalculator
- Returns cost breakdown (SQS/Lambda/Bedrock)
- Returns timeseries data grouped by hour

Refactored main.py:
- Simplified to use api_router
- Added exception handlers
- Added health check endpoint

All endpoints tested and working.

Tasks: BE-006, BE-007, BE-008, BE-009, BE-010 complete

src/api/deps.py

@@ -0,0 +1,39 @@
+"""API dependencies."""
+
+from typing import AsyncGenerator
+from uuid import UUID
+from fastapi import Depends, Header
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from src.core.database import AsyncSessionLocal
+from src.repositories.scenario import scenario_repository, ScenarioStatus
+from src.core.exceptions import NotFoundException, ScenarioNotRunningException
+
+
+async def get_db() -> AsyncGenerator[AsyncSession, None]:
+    """Dependency that provides a database session."""
+    async with AsyncSessionLocal() as session:
+        try:
+            yield session
+        finally:
+            await session.close()
+
+
+async def get_running_scenario(
+    scenario_id: str = Header(..., alias="X-Scenario-ID"),
+    db: AsyncSession = Depends(get_db),
+):
+    """Dependency that validates scenario exists and is running."""
+    try:
+        scenario_uuid = UUID(scenario_id)
+    except ValueError:
+        raise NotFoundException("Scenario")
+
+    scenario = await scenario_repository.get(db, scenario_uuid)
+    if not scenario:
+        raise NotFoundException("Scenario")
+
+    if scenario.status != ScenarioStatus.RUNNING.value:
+        raise ScenarioNotRunningException()
+
+    return scenario