feat: add QA dataset export for events, views, and KPIs (#742)

[Ejirowebfi]

closes #742

Summary

Implements #742 — adds repeatable QA dataset exports for events, materialized views, and KPIs, targeting a Stellar ledger range.

Changes

• src/qa_exporter.py — QAExporter class with export_events(), export_views(), export_kpis(), and run() methods
• scripts/export_qa_datasets.py — CLI script (--start-ledger, --end-ledger, --output-dir, --datasets)
• scripts/README_qa_export.md — documents output format, envelope schema, and usage
• tests/test_qa_exporter.py — 9 unit tests with mocked ledger range (all passing)

Usage

python scripts/export_qa_datasets.py --start-ledger 1000 --end-ledger 2000

Outputs events_1000_2000.json, views_1000_2000.json, kpis_1000_2000.json to exports/qa/.

Acceptance Criteria

- [x] Exports include raw events + materialized views
- [x] Can target a ledger range via --start-ledger / --end-ledger
- [x] Output format documented in scripts/README_qa_export.md

Testing

pytest tests/test_qa_exporter.py -v  # 9 passed

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