fix(helpers): add TTL cache to dir_size_gb to prevent blocking rglob walks

[boffin-dmytro]

Summary

• dir_size_gb() uses path.rglob("*") which recursively walks every file in a directory tree — on large model directories (tens of GB, thousands of files) this blocks the request thread on every call
• Added a per-path TTL cache (60s) so repeated calls from the resources, extensions, and storage endpoints return cached values instead of re-walking the filesystem
• Added invalidate_dir_size_cache() and wired it into purge_extension_data so the cache stays accurate after mutations

Test plan

• Hit /api/services/resources multiple times within 60s — second call should return instantly without re-walking disk
• Purge an extension via /api/extensions/{id}/data — subsequent resource calls should reflect the freed space
• Verify existing tests still pass (all tests mock dir_size_gb, so no test changes needed)

[Lightheartdevs]

TTL cache is the right shape for dir_size_gb. Reviewed the implementation:

• 60s TTL is reasonable — dashboard polls at ~5s intervals so this cuts rglob walks by an order of magnitude.
• invalidate_dir_size_cache() correctly called after shutil.rmtree in purge_extension_data so the next poll doesn't return stale size.
• clear_dir_size_cache() is a nice escape hatch but I didn't see it called anywhere — consider adding it to _clear_settings_caches if settings changes can move data dirs.

Minor note: the cache uses path.resolve() as key — good because it canonicalizes symlinks, but beware if callers pass relative paths from different cwds during a single process lifetime. Current callers all pass absolute paths, so fine for now.

The unrelated update-script-path resolution in routers/updates.py is a small bonus cleanup (collapses a confusing nested conditional). Ship.

The merge-base changed after approval.

[Lightheartdevs]

Re-submitting — prior approval review was dismissed (likely by an automation). TTL cache + invalidation pattern is correct, tests look good, 60s TTL is sensible for dashboard polling cadence. Details in the dismissed review.

The merge-base changed after approval.