DEV_GUIDE.md and the three READMEs still advertise Go 1.25.7 and
golangci-lint v2.7, but CI has since moved on:
- backend/go.mod declares go 1.26.5, and backend-ci.yml / release.yml /
security-scan.yml all resolve the toolchain via
`go-version-file: backend/go.mod` and then hard-assert
`go version | grep -q 'go1.26.5'`.
- backend-ci.yml pins golangci-lint to v2.9.
So a contributor following DEV_GUIDE.md installs a linter two minor
versions behind CI (different findings locally vs. in CI) and expects a
Go version that the workflow's own assertion step rejects.
Update all ten stale references, and note in the CI section which files
the Go version assertion lives in so future bumps don't miss one.
Docs only, no code or workflow changes.
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