chore: remove committed binaries and scratch dirs; tighten .gitignore

- Remove committed Go binaries:
    backend/bin/api-server (~18 MB)
    backend/cmd (~18 MB)
    backend/api/rest/cmd/api-server (~18 MB)
- Remove scratch / build output dirs from the repo:
    out/, cache/, test-results/
- Extend .gitignore to cover these paths plus playwright-report/
  and coverage/ so they don't drift back in.

Total artifact weight removed: ~54 MB of binaries + small scratch files.

out/build-info/0c2d00d4aa6f8027.json

@@ -1 +0,0 @@
-{"id":"0c2d00d4aa6f8027","source_id_to_path":{"0":"src/MockLinkToken.sol"},"language":"Solidity"}