notify resource owner when admin shares or unshares

[fabracht]

Summary

Closes #110.

• A resource owner is now notified when someone else (e.g. an admin acting on their behalf) shares or unshares their resource under scoped events; the owner is skipped when they performed the action themselves.
• Previously only the share path resolved the owner. The unshare path deleted _shares records with an empty ownership config, and the delete path precomputes event recipients from that config, so record_owner returned None and the owner was silently omitted on every unshare (single and cascade).
• Fix threads the real ownership config through the revoke path (share_revoke → clear_grant → delete_grants, plus write_grant/clear_all_resource_grants), so the owner is resolved while the resource record is still live.
• The resource-deletion cascade still resolves the owner to None (the resource is already gone by then), which is out of scope here; the grantee is still notified in that path.

Test plan

• new end-to-end test_unshare_by_admin_notifies_owner drives real Share/Unshare requests as an admin and asserts the emitted _shares delete event reaches both owner and grantee; verified it fails without the fix (owner missing) and passes with it
• existing share-event and cascade-delete recipient tests pass
• full mqdb-agent test suite green
• cargo make clippy clean