Implements slice 1 of docs/design/2026-07-19-client-diagnostics.md: the
versioned contract (schemas, fixtures, Go validator), storage-validated
diagnostics.uploads_enabled gate, account-scoped status endpoint, hardened
streaming multipart ingest with quota reservation and a receiving/ready/
failed report state machine, S3 streaming puts, acting-admin report API
(list/detail/download/delete with audit events), and the retention +
orphan-reconciliation cleanup task.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XppCCycoaskCsW7ja1fZct
