Add response compression and stream negotiation for API gateway

[Smartdevs17]

Context

API responses (invoice CSVs, analytics reports, export dumps) can exceed 10MB uncompressed, causing slow downloads and high egress costs.

Current Limitation/Problem

No response compression. Large invoice CSV exports take 30+ seconds to download on mobile connections.

Expected Outcome

Brotli/gzip compression negotiation via Accept-Encoding header, streaming responses for large payloads, and configurable compression level per endpoint.

Acceptance Criteria

• Compression middleware: Brotli (preferred, level 4 default) and gzip (fallback, level 6) via Accept-Encoding
• Compression level config: endpoint-level override via x-compression-level response header
• Minimum size threshold: only compress responses >1KB (skip tiny responses)
• Streaming: large CSV/JSON responses served with Transfer-Encoding: chunked
• Skip list: certain endpoints skip compression (streaming video, already-compressed binary)
• Performance: Brotli reduces JSON payload by 75%, CSV by 85% (measured with sample data)
• Edge case: client sends unsupported encoding (respond uncompressed with identity Content-Encoding)

Technical Scope

• backend/shared/middleware/compression.ts - compression negotiator middleware
• backend/shared/middleware/streaming.ts - streaming response middleware
• backend/config/ - endpoint-level compression configuration
• backend/gateway/ - API gateway compression config (if separate)
• backend/tests/ - compression integration and performance tests

[AlphaTechini]

@AlphaTechini has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

I would like to handle this issue

ℹ️ Repo Maintainers: To accept this application, review their application or assign @AlphaTechini to this issue.

[drips-wave]

Congratulations, @AlphaTechini! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

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