feat: scheduled developer report exports to object storage

[Ope4196]

PR: Scheduled Developer Report Exports to Object Storage

Summary

Adds a daily export pipeline that materialises usage_events into per-developer CSV and JSON artifacts stored in S3-compatible object storage, and exposes a signed download URL endpoint at GET /api/developers/exports.

This replaces the previous synchronous export approach which timed out on large date ranges.

Changes

New files

• migrations/0017_developer_exports.sql — developer_exports table with id, developer_id, format, s3_key, exported_at, expires_at and a composite index on (developer_id, exported_at DESC)
• src/services/reportExporter.ts — ReportExporterService, InMemoryExportStore, DeveloperExportStore interface, createReportExporterWorker worker factory
• src/services/reportExporter.test.ts — unit tests for service, store, and worker lifecycle

Modified files

• src/db/schema.ts — added developerExports Drizzle table definition, DeveloperExport and NewDeveloperExport types
• src/routes/developerRoutes.ts — added GET /exports route and extended DeveloperRoutesDeps with optional reportExporterService
• src/routes/developerRoutes.test.ts — added describe('GET /api/developers/exports') test block (5 cases)
• docs/scheduled-exports.md — updated to document the new table, route, TTL config, daily job interval, and in-memory test adapter

Test coverage

Test file	Cases
src/services/reportExporter.test.ts	8 (runDailyExports window, empty window, boundary, multi-dev, expired records, valid+expired mix, signed URL, worker lifecycle)
src/routes/developerRoutes.test.ts	5 new (401, 403, 200 with records, 200 empty, downloadUrl correctness)

Security

• Signed URLs expire per EXPORT_SIGNED_URL_TTL_SECONDS (default 900 s)
• S3 credentials are never returned in responses or logged
• Route scopes queries strictly to developer.user_id — no cross-tenant reads possible

closes #398