ci: bump golangci-lint to v2.13.1 to fix panic on Go 1.27 (#1561)

CI is red on `main` and on every open PR (including the Dependabot PRs
#1531, #1552–#1554) with:

```
panic: file requires newer Go version go1.27 (application built with go1.26)
golangci-lint exit with code 2
```

## Cause

`actions/setup-go` with `go-version: 'stable'` started resolving to
**go1.27.0**. The pinned golangci-lint **v2.11.4** release binary is
built with **go1.26**, and its `go/types` refuses to typecheck source
files declaring a newer language version. Nothing in the failing PRs is
at fault.

## Fix

Bump to **golangci-lint v2.13.1**, whose release binaries are built with
go1.27, so linting works under `stable` again.

That bump also pulls in newer linter versions, which surface 57 findings
this tree did not have before. Handled as follows:

| Linter | Count | Resolution |
|---|---|---|
| `goconst` | 50 | 39 were repeated literals in `_test.go` files —
goconst started counting those in v2.12.0 (golangci-lint#6480 exposed
`ignore-tests`). Fixture data like `"1.0.0"` (89 occurrences) is not a
missing constant, so `goconst.ignore-tests: true`. The other 11 were
real duplication in production code (OpenAPI tags, the `bearer` security
scheme name, `$ref`, `http`/`https`) and became named constants. |
| `gosec` G710 | 1 | **True positive, fixed.** See below. |
| `staticcheck` SA1019 | 6 | Go 1.26 deprecated `ecdsa.PublicKey.X/Y`
and `PrivateKey.D`. Annotated with the same targeted `nolint` +
rationale already used for `ScalarBaseMult` in
`cmd/publisher/auth/common.go`. |

### The open redirect is real

`TrailingSlashMiddleware` copied the entire `*url.URL` before rewriting
the path. For an absolute-form request URI — legal for a Go HTTP server
to parse, e.g. `GET http://evil.com/foo/ HTTP/1.1` — `r.URL.Scheme` and
`r.URL.Host` are populated, and they were echoed straight back in the
`Location` header. The redirect target is now built from the cleaned
path and query alone. The existing `path.Clean` hardening for
GHSA-v8vw-gw5j-w7m6 (protocol-relative `//evil.com/`) is unchanged.

### Also

Switched `gomodguard` → `gomodguard_v2`; the former is deprecated as of
v2.12.0 and was emitting a warning on every run.

## Verification

- `golangci-lint v2.13.1` against this tree: **0 issues**, no
deprecation warnings
- `make validate`: all validations passed
- `go build ./...` and tests for every touched package pass (DB-backed
tests need Postgres; left to CI)

## Note on migrating the ecdsa key paths

Moving those six sites to `PublicKey.Bytes` /
`ParseUncompressedPublicKey` / `crypto/ecdh` touches key parsing and
signature verification. It should be its own reviewed PR rather than
riding along on a CI unblock. Happy to open one.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

internal/api/server.go

@@ -5,6 +5,7 @@ import (
 	"encoding/json"
 	"log"
 	"net/http"
+	"net/url"
 	"path"
 	"strings"
 	"time"
@@ -72,11 +73,18 @@ func TrailingSlashMiddleware(next http.Handler) http.Handler {
 	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		// Only redirect if the path is not "/" and ends with a "/"
 		if r.URL.Path != "/" && strings.HasSuffix(r.URL.Path, "/") {
+			// Build the target from the path and query alone, dropping any
+			// scheme/host the client may have supplied in an absolute-form
+			// request URI ("GET http://evil.com/foo/ HTTP/1.1"), which would
+			// otherwise be echoed back as an off-site redirect.
+			//
 			// path.Clean both removes the trailing slash and collapses any
 			// leading "//" to "/", which prevents an open-redirect via a
 			// protocol-relative path like "//evil.com/" (GHSA-v8vw-gw5j-w7m6).
-			newURL := *r.URL
-			newURL.Path = path.Clean(r.URL.Path)
+			newURL := url.URL{
+				Path:     path.Clean(r.URL.Path),
+				RawQuery: r.URL.RawQuery,
+			}
 
 			// Use 308 Permanent Redirect to preserve the request method
 			http.Redirect(w, r, newURL.String(), http.StatusPermanentRedirect)