Python AsyncIO & FastAPI High-Throughput Microservice Architect

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Act as a Principal Python Engineer specializing in AsyncIO, high-throughput FastAPI microservices, and asynchronous event loops.

Write a clean, non-blocking, production-ready FastAPI service module designed to handle 5,000+ concurrent requests.

### SERVICE REQUIREMENTS:
- Microservice Purpose: [MICROSERVICE_PURPOSE]
- External I/O Operations (Database queries, third-party API fetches, file writes): [EXTERNAL_IO_CALLS]
- Concurrency & Rate Limiting Requirements: [CONCURRENCY_TARGETS]
- Data Validation Models: [INPUT_AND_OUTPUT_DATA]

### IMPLEMENTATION DELIVERABLES:
1. **Pydantic v2 Schemas**:
   - Strict input validation schemas with custom field validators and serialization performance optimizations.
2. **AsyncIO Parallel Execution Pattern**:
   - Using `asyncio.gather` or `asyncio.TaskGroup` to trigger multiple independent I/O tasks concurrently without blocking the main event loop.
3. **Database & HTTP Connection Pooling**:
   - Lifespan context manager (`@asynccontextmanager`) managing persistent `httpx.AsyncClient` and async database pool (asyncpg / SQLModel).
4. **Semaphore & Concurrency Throttling**:
   - Using `asyncio.Semaphore` to prevent overwhelming third-party rate limits during traffic spikes.
5. **Comprehensive Error Handling & Middleware**:
   - Custom exception handlers catching timeouts and returning structured RFC 7807 JSON error objects with correlation IDs.

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