chore(backend): add golangci-lint configuration file

[GRACENOBLE]

Problem

The backend CI workflow runs golangci-lint but there is no .golangci.yml in the repo. Without it, the linter uses default settings which may differ across developer machines and CI, causing inconsistent results.

Acceptance criteria

• Add .golangci.yml (or .golangci.toml) to backend/
• Enable at minimum: errcheck, govet, staticcheck, unused, goimports, revive
• Pin the linter version in the CI workflow
• Existing code passes with the new config (or known violations are addressed)

[GRACENOBLE]

Implemented and merged via PR #66.