feat: implement ledger-range export generator

[ifygreg01-best]

Summary

Adds a ledger-range export generator to the data-processing service to aid maintainers during incident debugging.

Changes

• src/ledger_export.py — LedgerRangeExporter class: validates range, exports raw ContractEvent rows and normalized ProjectView/ProjectContributor/ProjectMilestone state into a single deterministic JSON file
• scripts/export_ledger_range.py — CLI entry point
• tests/test_ledger_export.py — 20 tests (all passing)
• scripts/README_ledger_export.md — documentation

Export format

{
  "metadata": { "startLedger": 1000, "endLedger": 2000, "exportTimestamp": "...", "exportVersion": "1" },
  "raw": [...],
  "normalized": { "project_views": [...], "project_contributors": [...], "project_milestones": [...] }
}

Testing

All 20 new tests pass. No regressions in existing suite.

Closes #883

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[Cedarich]

@ifygreg01-best fix ci