032: Phase 1-2 — setup deps + AsyncAPIClient extend + client_registry + executors

Phase 1 (Setup):
- T001: requirements-dev.txt with pytest-httpx
- T002: aiofiles added to requirements.txt
- T003: aiosmtplib added to requirements.txt
- T004: EnvironmentConfig extended (connection_pool_size, etc.)
  + AppAsyncRuntimeConfig created (executor workers, shutdown)

Phase 2 (Foundational):
- T007: AsyncAPIClient extended — semaphore parameter, request() method
- T008a: SupersetClientRegistry — singleton per-env client/semaphore/lock
- T008b: run_blocking helper + bounded executors (db/file/git)

RATIONALE: httpx.AsyncClient replaces requests.Session; singleton
registry ensures global per-env semaphore; named executors prevent
thread pool exhaustion.
REJECTED: asyncio.to_thread (default executor, no backpressure);
per-request clients (lose pooling); dual-stack (rejected at clarify).

backend/requirements.txt

@@ -60,3 +60,5 @@ ruff>=0.11.0
 sqlparse>=0.5.0
  lingua-language-detector==2.1.1
 testcontainers[postgres]>=4.0
+aiofiles>=24.1.0
+aiosmtplib>=3.0.2