feat(club-005): club registered users: account linking and automatic recovery

work/artifacts/CLUB-005/documenter.md

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+# CLUB-005 — Documentation
+
+## Endpoints added/modified
+
+### PATCH /club/me
+Link the anonymous Club member (identified by device token cookie) to the authenticated user account.
+- **Auth**: session required
+- **Response**: `{ member, linked: boolean, config }`
+
+### POST /club/merge
+Merge an anonymous member into the authenticated user's registered member.
+- **Auth**: session required
+- **Body**: `{ anonymousMemberId: uuid }`
+- **Response**: `{ member, mergedId, config }`
+- The anonymous member's balance is added to the registered member
+- All devices are transferred to the registered member
+- The anonymous member is marked as `merged`