docs(release): update contracts and docs for functional-completion changes

Add the new endpoints to contracts/openapi.yaml and docs/05-api-contract.md
(return-preview, the four rule-specific data-quality resolution endpoints,
search, integration status, scheduled-scan), document the role matrix and
the audit before/after exposure in docs/12-security-and-audit.md, document
each rule type's actual resolution flow in docs/07-data-quality.md
(including the deliberate evidence-fingerprint simplification and the
reopened_from/previous_decision recurrence link), document the preview/
commit relationship in docs/08-return-workflow.md, and update README.md's
scope/integration-status/quality-gate sections to match what's actually
implemented and verified now. Also drops docs/05-api-contract.md's mention
of GET /api/v1/system/status, which was never implemented.

docs/08-return-workflow.md

@@ -1,5 +1,16 @@
 # Vehicle-return workflow
 
+## Preview
+
+`POST /api/v1/bookings/{public_ref}/return-preview` takes the same request body as the
+commit endpoint below and runs the identical evaluation (`evaluate_return()`) with no
+writes, no audit event and no outbox event — it exists so the UI's review step shows the
+server's actual answer instead of guessing the outcome client-side. It returns the
+canonical and submitted odometer readings, whether the submission is a regression, the
+resulting vehicle status with a human-readable reason, whether a quality issue would be
+created, and next-booking risk. `register_vehicle_return` (below) calls the same
+`evaluate_return()` function, so preview and commit cannot drift apart.
+
 ## Input
 
 - booking public reference;