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/crypto/crypto.go

@@ -1,13 +1,13 @@
 // Package crypto provides key management and encryption helpers for MCIAS.
 //
 // Security design:
-// - All random material (keys, nonces, salts) comes from crypto/rand.
-// - AES-256-GCM is used for symmetric encryption; the 256-bit key size
-//   provides 128-bit post-quantum security margin.
-// - Ed25519 is used for JWT signing; it has no key-size or parameter
-//   malleability issues that affect RSA/ECDSA.
-// - The master key KDF uses Argon2id (separate parameterisation from
-//   password hashing) to derive a 256-bit key from a passphrase.
+//   - All random material (keys, nonces, salts) comes from crypto/rand.
+//   - AES-256-GCM is used for symmetric encryption; the 256-bit key size
+//     provides 128-bit post-quantum security margin.
+//   - Ed25519 is used for JWT signing; it has no key-size or parameter
+//     malleability issues that affect RSA/ECDSA.
+//   - The master key KDF uses Argon2id (separate parameterisation from
+//     password hashing) to derive a 256-bit key from a passphrase.
 package crypto
 
 import (