Bound storage usage cache task runtime

[kvnloo]

Closes #11816

Problem

update_storage_usage_cache can hold a Celery worker indefinitely when its paginated storage query stalls.

Approach

• set a 270-second soft limit and 300-second hard limit on the task
• log the affected target_guid with traceback when the soft limit fires
• re-raise SoftTimeLimitExceeded so Celery preserves timeout semantics

Verification

• focused regression tests: 2 passed
• flake8 api/caching/tasks.py api/caching/test_tasks.py
• python -m compileall -q api/caching/tasks.py api/caching/test_tasks.py
• git diff --check origin/develop...HEAD
• strict RED → GREEN verification, including sabotage runs for both the task limits and timeout logging behavior
• independently reviewed with no blocking safety, security, or test-adequacy findings

Risk

The change only affects the storage-usage cache task after 270 seconds of execution. The existing disabled-cache early return remains outside the timeout handler, and timed-out work is logged then propagated rather than silently swallowed.

AI assistance was used for implementation and test review; the contributor inspected the diff and ran the verification listed above.