service: generated by-id handler returns 500 {defined:false} for a missing row instead of a defined 404

[rickylabs]

Found during the #1250 composed verification (live scaffold at PR #1256's head, users service on its assigned port). With an empty users table:

GET /api/users/1 → 500 {"defined":false,"code":"INTERNAL_SERVER_ERROR","status":500,"message":"User 1 not found"}
GET /api/users/abc → 400 (validation — correct)
GET /api/users?page=1&limit=5 → 200 (coercion — correct post-#1250)

The handler's not-found throw is not a defined oRPC error, so it surfaces as a 500. The docs-page default request for the by-id GET therefore errors on any fresh project (compounded by the seed being a no-op — filed separately).

Acceptance

• Generated by-id/update/delete handlers throw a defined NOT_FOUND (404) for missing rows
• The OpenAPI projection documents the 404 for those operations
• A scaffold-level test covers missing-row → 404 (not 500) for each generated by-id operation

[rickylabs]

Milestone rollover (owner-directed, 2026-08-06): moved from the former mixed milestone 24, now renamed 0.0.7 for the Frontend Contribution Layer, into the newly created 0.0.6. This open item is not #922 or an epic:frontend-contrib child, so it belongs in the immediate post-0.0.5 follow-up release. Closed historical assignments were not changed.