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docs/dependency-security.md

-# Dependency Security
-
-This document describes how dependency and toolchain security is managed in this repo.
-
-## Go Toolchain Policy (Pinned to 1.25.5)
-
-The Go toolchain is pinned to 1.25.5 to address known security issues.
-
-Locations that MUST stay aligned:
-- `backend/go.mod`: `go 1.25.5` and `toolchain go1.25.5`
-- `Dockerfile`: `GOLANG_IMAGE=golang:1.25.5-alpine`
-- Workflows: use `go-version-file: backend/go.mod` and verify `go1.25.5`
-
-Update process:
-1. Change `backend/go.mod` (go + toolchain) to the new patch version.
-2. Update `Dockerfile` GOLANG_IMAGE to the same patch version.
-3. Update workflows if needed and keep the `go version` check in place.
-4. Run `govulncheck` and the CI security scan workflow.
-
-## Security Scans
-
-Automated scans run via `.github/workflows/security-scan.yml`:
-- `govulncheck` for Go dependencies
-- `gosec` for static security issues
-- `pnpm audit` for frontend production dependencies
-
-Policy:
-- High/Critical findings fail the build unless explicitly exempted.
-- Exemptions must include mitigation and an expiry date.
-
-## Audit Exceptions
-
-Exception list location: `.github/audit-exceptions.yml`
-
-Required fields:
-- `package`
-- `advisory` (GHSA ID or advisory URL from pnpm audit)
-- `severity`
-- `mitigation`
-- `expires_on` (recommended <= 90 days)
-
-Process:
-1. Add an exception with mitigation details and an expiry date.
-2. Ensure the exception is reviewed before expiry.
-3. Remove the exception when the dependency is upgraded or replaced.
-
-## Frontend xlsx Mitigation (Plan A)
-
-Current mitigation:
-- Use dynamic import so `xlsx` only loads during export.
-- Keep export access restricted and data scope limited.
-
-## Rollback Guidance
-
-If a change causes issues:
-- Go: revert `backend/go.mod` and `Dockerfile` to the previous version.
-- Frontend: revert the dynamic import change if needed.
-- CI: remove exception entries and re-run scans to confirm status.