server: own standalone WebRTC sweep

[yisding]

Problem

Standalone WebRTC shutdown created its final forced SessionManager.stop_all() sweep with raw asyncio.create_task(). A hard timeout can intentionally return while cancellation-resistant Session teardown is still unwinding, so this is exactly the kind of survivor that requires explicit ownership.

Changes

• create the final Session sweep through a named RuntimeTaskScope
• preserve the existing hard deadline, timeout warning, aggregate report handling, and error precedence
• release the standalone scope when the sweep settles within the shutdown window
• assert the sweep executes under the stable task name easycat-standalone-webrtc-session-sweep
• reduce the reviewed raw-task-spawn inventory from 33 to 32
• document ownership across a hard-timeout return

Why / impact

A timed-out final sweep remains strongly owned while it reacts to cancellation instead of becoming an anonymous background task. Completed failure reports continue to retain Session ledgers and surface the same shutdown error.

Root cause

The standalone server's final sweep used the hard-timeout helper's legacy background retention but had not been migrated to the named task ownership model in WS2.5.

Checks

• standalone shutdown/source-inventory focused tests — 12 passed across focused runs
• full WebRTC lifecycle file outside the sandbox — 45 passed / 33 skipped
• Ruff on changed Python files — passed
• mypy on webrtc_routes.py — passed
• pre-commit hooks — passed

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