diagram sharing phase 3 and optimistic-locking retry fix

[fabracht]

Summary

• Diagram sharing phase 3: recipient-scoped event confidentiality, with scoped event recipients resolved at write time so cascade deletes reach the owner and grantees.
• Optimistic locking: update/delete retry on CAS conflict (converges to field-level last-writer-wins); the fjall backend's CAS is now atomic (was a check-then-act that could silently drop a concurrent write).

Test plan

• cargo make dev (format + clippy-pedantic + full suite, 1047 tests) green
• new concurrency tests: update convergence and delete/index consistency

[fabracht]

Superseded by #87. Phase-3 work already landed on main via #83, so this branch only duplicated it; #87 is the clean CAS-only change off current main (mqdb-core 0.7.3, mqdb-agent 0.8.8, mqdb-cli 0.8.9).