Merge RB-18 — key IdempotencyStore on caller plus idem key

BIO-018: the store was a process-global dictionary keyed on the client-supplied
Idempotency-Key alone, so one caller could replay another caller's key and
receive their cached response. The key is now scoped with the caller SubjectId.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

# Conflicts:
#	docs/project/refactor-backlog-setup/refactor-backlog/99-backlog.md
#	libs/shared/docs/behaviour-spec.mdx

libs/shared/docs/behaviour-spec.mdx

@@ -21,7 +21,7 @@ tested where._
 Every bullet below is a real test name from the suite — an `it()` title (frontend) or a test
 method name (backend), read as a sentence. Nothing here is hand-written prose: this page
 **is** the suite, reshaped for a business reader. 460 frontend behaviours across
-9 contexts; 236 backend behaviours across 41 test
+9 contexts; 237 backend behaviours across 41 test
 classes.
 
 ## Frontend (by context)
@@ -1064,6 +1064,7 @@ classes.
 
 - Replaying the same idempotency key returns the same reference not a new one
 - Different idempotency keys are independent submissions
+- A caller replaying another callers idempotency key does not get their cached result
 - A rejected submission replays the same rejection not a retry
 
 ### IntakeRuleTests