feature: publish a JSON Schema for the CleanReport payload (follows #21)

[JohnnyWilson16]

Problem

#21 adds JSON export for CleanReport. Consumers integrating the audit trail into pipelines also need a stable schema to validate that JSON against, so downstream tooling can rely on the shape.

Follows #21 (the JSON export). Best done once the export format is settled.

Expected outcome

A published JSON Schema for the report payload, plus a small helper (e.g. report.to_json_schema() or a shipped schemas/clean_report.schema.json).

Relevant files

• src/freshdata/report.py (or a new schemas/ file)
• docs (audit-trail page)
• tests/ (validate real report.to_dict() output against the schema)

Acceptance criteria

• The schema validates real report.to_dict() / JSON-export output.
• A test asserts a representative report validates against it.
• Documented under the audit-trail docs.

Difficulty / skills

🚀 mid — typing, JSON Schema (jsonschema is already a dev dep).