Add `response.rs` consistency check for error payload shape across all wired handlers

[fejilaup-cloud]

Labels: bug, api-server
Priority: Medium
Estimated Time: 1.5 hours

Description:
Once #36-#47 wire real contract errors through, and #49 adds an error-to-status mapping table, confirm response.rs's error envelope (fields like code, message, details) is applied uniformly rather than each handler constructing its own ad hoc error body.

Tasks:

• Define one canonical error response struct in response.rs
• Update all handlers to use it exclusively
• Add a test asserting every error response across every endpoint matches the canonical shape

[Peculiarr7]

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