server: own capacity drain tasks

[yisding]

Problem

CapacityGate.drain() started graceful stop workers, its cancellation-shielded finisher, and per-session escalations with raw asyncio task APIs. Only the outer finisher had an instance ledger, while cancellation-resistant descendants depended indirectly on the grandfathered module-global hard-timeout ledger.

Changes

• move every CapacityGate drain worker into one named RuntimeTaskScope
• keep graceful, finish, and escalation task identities distinct
• retain cancellation-resistant force work directly under the gate until settlement
• add failure-path assertions for caller cancellation and hard-timeout survivors
• reduce the production raw-task baseline from 29 to 26
• document the gate's lifecycle ownership

Why / impact

The drain's timing and swallow/propagate behavior stay unchanged, but task lifetime is now explicit and local to the gate. Caller cancellation and force-deadline expiry can no longer leave per-session teardown dependent only on a module-global anchor.

Checks

• .venv/bin/python -m pytest tests/server/test_capacity_gate_drain.py -q (63 passed)
• .venv/bin/python -m pytest tests/server/test_websocket_runtime_drain.py tests/server/test_voice_server_lifecycle.py tests/server/test_webrtc_routes.py tests/ratchets/test_source_inventory.py -q (51 passed, 28 skipped)
• .venv/bin/ruff check src/easycat/server/transports.py tests/server/test_capacity_gate_drain.py
• .venv/bin/ruff format --check src/easycat/server/transports.py tests/server/test_capacity_gate_drain.py
• .venv/bin/mypy src/easycat/server/transports.py
• pre-commit hooks

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