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

pyproject.toml

@@ -10,6 +10,7 @@ dependencies = [
     "asyncpg>=0.31.0",
     "fastapi>=0.110.0",
     "pydantic>=2.7.0",
+    "pydantic-settings>=2.13.1",
     "tiktoken>=0.6.0",
     "uvicorn>=0.29.0",
 ]