Complete implementation: e2e tests, gofmt, hardening

- Add test/e2e: 11 end-to-end tests covering full login/logout,
  token renewal, admin account management, credential-never-in-response,
  unauthorised access, JWT alg confusion and alg:none attacks,
  revoked token rejection, system account token issuance,
  wrong-password vs unknown-user indistinguishability
- Apply gofmt to all source files (formatting only, no logic changes)
- Update .golangci.yaml for golangci-lint v2 (version field required,
  gosimple merged into staticcheck, formatters section separated)
- Update PROGRESS.md to reflect Phase 5 completion
Security:
  All 97 tests pass with go test -race ./... (zero race conditions).
  Adversarial JWT tests (alg confusion, alg:none) confirm the
  ValidateToken alg-first check is effective against both attack classes.
  Credential fields (PasswordHash, TOTPSecret*, PGPassword) confirmed
  absent from all API responses via both unit and e2e tests.
  go vet ./... clean. golangci-lint v2.6.2 incompatible with go1.26
  runtime; go vet used as linter until toolchain is updated.

internal/auth/auth.go

@@ -1,15 +1,15 @@
 // Package auth implements login, TOTP verification, and credential management.
 //
 // Security design:
-// - All credential comparisons use constant-time operations to resist timing
-//   side-channels. crypto/subtle.ConstantTimeCompare is used wherever secrets
-//   are compared.
-// - On any login failure the error returned to the caller is always generic
-//   ("invalid credentials"), regardless of which step failed, to prevent
-//   user enumeration.
-// - TOTP uses a ±1 time-step window (±30s) per RFC 6238 recommendation.
-// - PHC string format is used for password hashes, enabling transparent
-//   parameter upgrades without re-migration.
+//   - All credential comparisons use constant-time operations to resist timing
+//     side-channels. crypto/subtle.ConstantTimeCompare is used wherever secrets
+//     are compared.
+//   - On any login failure the error returned to the caller is always generic
+//     ("invalid credentials"), regardless of which step failed, to prevent
+//     user enumeration.
+//   - TOTP uses a ±1 time-step window (±30s) per RFC 6238 recommendation.
+//   - PHC string format is used for password hashes, enabling transparent
+//     parameter upgrades without re-migration.
 package auth
 
 import (
@@ -168,10 +168,10 @@ func parsePHC(phc string) (ArgonParams, []byte, []byte, error) {
 // A ±1 time-step window (±30s) is allowed to accommodate clock skew.
 //
 // Security:
-// - Comparison uses crypto/subtle.ConstantTimeCompare to resist timing attacks.
-// - Only RFC 6238-compliant HOTP (HMAC-SHA1) is implemented; no custom crypto.
-// - A ±1 window is the RFC 6238 recommendation; wider windows increase
-//   exposure to code interception between generation and submission.
+//   - Comparison uses crypto/subtle.ConstantTimeCompare to resist timing attacks.
+//   - Only RFC 6238-compliant HOTP (HMAC-SHA1) is implemented; no custom crypto.
+//   - A ±1 window is the RFC 6238 recommendation; wider windows increase
+//     exposure to code interception between generation and submission.
 func ValidateTOTP(secret []byte, code string) (bool, error) {
 	if len(code) != 6 {
 		return false, nil