perf: reuse known instantiated storages in SharedMounts

[salmart-dev]

Summary

Avoids querying over and over the DB to initiate a SharedMount for a storage that is already known. This stops N+1 issues when using instanceOfStorage on SharedStorage for the same root storage through different SharedStorage instances.

TODO

• Double-check that getting the storage ID in the way I did is okay and behaves as expected with multiple wrappers
• Add steps to reproduce and description of what the root/source path handling does

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI