Export Reversals

[hayesZach]

Closes CSF-890
Refs CSF-881
Refs CSF-878
Refs CSF-700

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Note

Medium Risk
Adds a new data-export surface and increases max page size, which could impact performance/memory and data exposure if permissions are misconfigured, though it is gated by PermissionExport and covered by tests.

Overview
Adds a new GET /api/v1/reversals/export endpoint (behind PermissionExport) that returns the same filtered/paginated reversal list as GET /, but encoded as CSV with a fixed header set.

The export supports steam_id, marketplace_slug, limit, and cursor query params, raises the export limit ceiling to 50k rows, and surfaces pagination via an X-Next-Cursor response header. Extensive new tests validate CSV encoding (including null fields), filtering, pagination, limit validation, and missing-context error handling.

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[linear]

CSF-890 Export Reversals

CSF-881 Implement Endpoints

CSF-878 Reversals Endpoints

CSF-700 Add Comprehensive Tests