Replace Twisted with aiohttp and native asyncio

[sandhose]

Summary

• Replace Twisted HTTP server (Resource/Site) with aiohttp.web
• Replace Twisted HTTP client (Agent) with aiohttp.ClientSession
• Replace twisted.names SRV resolution with dnspython
• Replace reactor.run() with asyncio.run()
• Replace LoopingCall with asyncio.create_task() + sleep loops
• Migrate all 20 servlets from Resource classes to async handler functions
• Rewrite federation agent, IP blacklisting, and TLS for aiohttp
• Delete custom Twisted type stubs (16 .pyi files)
• Migrate tests from trial to pytest + pytest-asyncio
• Split CI into separate unit test and integration test jobs
• Fix mypy errors from aiohttp migration (proper type annotations)

Known limitations

• pyOpenSSL kept for replication peer cert CN extraction
• test_bind_notYourMxid may be flaky due to aiosmtpd startup race in integration tests

Test plan

• uv run pytest tests/ — 100 tests pass
• uv run pytest --pyargs matrix_is_tester — 37/37 pass
• uv run mypy — no errors
• No twisted or zope.interface imports remain
• CI green

Part 8 of 9 in the modernisation series. Builds on #627.