feat: surface the reporting schema version a server reports

[dannash100]

• Currency grading (RPT#currency) reads the schema version a server reports, and nothing carried it past the raw status extras.
• Status::reporting_schema_version() pulls the reportingSchemaVersion extra; the snapshot API gains reporting_schema alongside the other reported versions.
• Shown as a curated field on StatusSnapshot, hidden until a server reports one.
• Surfacing only. Grading current/behind/unknown waits on the Tamanu fact for the applied schema (U1, feat(public-server): U1: spec the reporting-schema pipeline #525).