phase 4: health enrichment, make help & golangci-lint config

[YASSERRMD]

Summary

• Health enrichment: GET /v1/health now returns log_backend, signer_type, and timestamp in addition to status and version; WithBackendInfo() setter wires these from main.go
• make help: all Makefile targets now carry ## description comments; make help prints a colour-coded usage table
• .golangci.yml: standardizes linting with errcheck, staticcheck, gosimple, ineffassign, unused, gofmt, misspell, unconvert, and bodyclose; test files are exempt from errcheck

Test plan

• GET /v1/health response includes log_backend, signer_type, and timestamp fields
• make help prints all targets with descriptions
• make lint passes with golangci-lint installed (or gracefully skips if absent)
• .golangci.yml is picked up automatically by golangci-lint in the project root