feat(api): return rfc 9457 problem details for errors

[TheMeinerLP]

Summary

All error responses of the Otis API now use RFC 9457 Problem Details (application/problem+json). This prepares the upcoming player settings API (stacked PR), which needs stable error types.

• Errors carry a stable type URI (https://otis.onelitefeather.net/problems/<slug>), title, status and, where useful, detail. Generic HTTP errors use about:blank with the status reason phrase as title.
• Bean validation failures are reported as constraint-violation with a violations[] extension member.
• When tracing is enabled (OTEL_TRACES_EXPORTER != none), every problem carries a traceId member to look the request up in the tracing backend.
• Unhandled exceptions become a 500 problem without internals and are logged once at ERROR.
• The OpenAPI spec documents ProblemDetail responses for every operation. The client is generated from the new java-client/specs/otis-api-1.1.0.yml (1.0.1 stays in place) and gains ProblemDetails.from(ApiException).
• CLAUDE.md now states that the client uses Jackson and describes the problem details.
• OpenSpec change add-problem-details (proposal, spec, design, tasks) and the OpenSpec scaffolding are included.

Compatibility (old plugins)

• Paths, methods, success status codes, success bodies and error status codes of /otis/** and /search/** do not change. Only the error body and its Content-Type change. Released velocity-plugin builds only evaluate status codes (404/500).
• ExistingEndpointsCompatibilityTest was written first and verified against unchanged main. It still passes after the switch.
• javap -public of the generated client before and after: additions only (ProblemDetail, ProblemViolation and their builders, ProblemDetails).
• velocity-plugin compiles and shadowJar builds without source changes.

Deviations / notes

• micronaut-problem-json 3.9.0 is based on org.zalando:problem. Its error processor is deprecated, so the body provider is replaced, and extension members are flattened to the top level as RFC 9457 requires.
• micronaut-tracing-opentelemetry-http marks every server span with status >= 400 as ERROR. That differs from the OTel semantic conventions, and there is no switch for it. The spec was narrowed to "problem rendering does not touch the span" instead of replacing the built-in tracing filter.
• Integration tests run on in-memory H2 (application-test.yml, CI has no Docker) with transactional = false, because two transaction managers make micronaut-test's transaction listener ambiguous.

Test plan

• ./gradlew clean build: 50 tests, 0 failures, 1 skipped (pre-existing disabled OtisTest)
• Characterization tests green on main and on this branch
• Client javap diff contains additions only; velocity-plugin shadowJar builds unchanged

https://claude.ai/code/session_012BmpdMdQ5wwagagQvfcq8S

[github-actions]

Test results

 33 files   33 suites   38s ⏱️
 50 tests  49 ✅ 1 💤 0 ❌
150 runs  147 ✅ 3 💤 0 ❌

Results for commit bf6086d.

♻️ This comment has been updated with latest results.