fix: surface missing reviewer harness

[Vaibhaav-Tiwari]

Summary

• map missing reviewer harness binaries to a typed 422 REVIEWER_BINARY_NOT_FOUND response with the original cause preserved
• record failed review launches as failed review runs so the UI has a durable trace instead of an empty gap
• allow retrying the same commit after a failed review run by excluding failed runs from the review-run uniqueness gate

Closes #303

Tests

• cd backend && go test ./internal/review
• cd backend && go test ./internal/httpd/controllers
• cd backend && go test ./internal/storage/sqlite ./internal/storage/sqlite/store