docs: add CONTRIBUTING.md, update README and .env.example - Add CONTR…

[alamuoyeemmanuel7-create]

summary

Adds HTTP response compression to the NestJS backend using the compression package. All JSON responses larger than 1 KB are now gzip-compressed before being sent to the client, reducing payload sizes by 60–80% on slow connections.

API responses were being sent uncompressed. For clients on mobile or slow networks this wastes bandwidth and increases latency, especially on list endpoints that return multiple invoices or pool positions.

Changes
backend/src/main.ts — registers compression() as an Express middleware globally, before all route handlers and exception filters. A custom filter function honours the x-no-compression request header and falls back to the standard compressible MIME-type check
backend/package.json — adds compression@1.7.4 to dependencies and @types/compression@1.7.5 to devDependencies (both pinned)
backend/src/types/compression.d.ts — minimal ambient type declaration so the TypeScript build passes before npm ci is run in a fresh environment
.env.example — documents two new optional environment variables with inline comments
Environment variables
Variable Default Description
COMPRESSION_THRESHOLD 1024 Minimum response size in bytes before compression is applied
COMPRESSION_LEVEL -1 zlib level 0–9, or -1 for the zlib default (level 6)
Behaviour notes
Responses below the threshold are sent as-is — no CPU overhead for small payloads
Content-Encoding: gzip is set automatically by the middleware on compressed responses
Streaming responses are not affected — the default Z_SYNC_FLUSH strategy forwards compressed chunks as they are written rather than buffering the full response
Any client or proxy can opt out per-request by sending x-no-compression: 1
Testing

closes #15