feat(cdn): add CDN asset pipeline module — upload, purge, origin config
- types.ts: CdnAsset, PurgeRequest, OriginConfig + 3 Zod schemas
- repository.ts: asset CRUD, purge request tracking, origin config management
- routes.ts: 9 endpoints (asset upload/list/get/delete, purge, origin config CRUD)
- cdn.test.ts: 15 schema validation tests
- Supports categories (image/video/font/script/style/document/other)
- SHA-256 content hash tracking, size limits, TTL-based purge requests
- Cosmos containers: cdn_assets, cdn_purge_requests, cdn_origin_configs
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