Expand Thingifier 4xx write status handling

[eviltester]

Summary

• map application validation failures to 422 Unprocessable Content
• map repository state conflicts such as duplicate primary keys and max instance limits to 409 Conflict
• add typed store write exceptions so application error mapping is semantic rather than string-based
• update generated route docs/OpenAPI response metadata for write routes
• update store, application, mapper, API regression, relationship, and docgen tests

Verification

• mvn -pl ercoremodel,thingifier -am verify
• git diff --check

Closes #73

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