lint: adopt org-standard .golangci.yml and golangci-lint v2.12.2 (closes #14)

The old backend/.golangci.yml declared version "2" but used v1 schema
keys, so under v2 it never validated and its thresholds were inert: the
linter ran at defaults. Replace it verbatim with the org-standard file,
repin the Dockerfile.backend lint stage to golangci-lint v2.12.2, and
assert the config's sha256 as the first step of the backend lint target
so it cannot silently drift again -- a local hash check, no network.
Fix every finding the standard config surfaces in the Go source: wrap
over-long lines, drop a dead //nolint:wsl, hoist the repeated test IP
literals in middleware_test.go to named constants (goconst), and switch
its request to NewRequestWithContext (noctx). TODO.md updated.

Model: opus-4-8

backend/internal/middleware/middleware_test.go

@@ -9,6 +9,15 @@ import (
 	"sneak.berlin/go/netwatch/internal/middleware"
 )
 
+const (
+	// loopbackPeer is a remote address inside the trusted-proxy allowlist.
+	loopbackPeer = "127.0.0.1:5000"
+	// forwardedIP is the client address presented via X-Forwarded-For.
+	forwardedIP = "203.0.113.7"
+	// realIP is the client address presented via X-Real-IP.
+	realIP = "203.0.113.9"
+)
+
 func mustPrefixes(t *testing.T, cidrs ...string) []netip.Prefix {
 	t.Helper()
 
@@ -41,32 +50,32 @@ func clientIPCases() []clientIPCase {
 	return []clientIPCase{
 		{
 			name:       "trusted proxy uses forwarded-for",
-			remoteAddr: "127.0.0.1:5000",
-			xff:        "203.0.113.7",
-			want:       "203.0.113.7",
+			remoteAddr: loopbackPeer,
+			xff:        forwardedIP,
+			want:       forwardedIP,
 		},
 		{
 			name:       "trusted proxy uses left-most of chain",
 			remoteAddr: "10.1.2.3:5000",
-			xff:        "203.0.113.7, 10.1.2.3",
-			want:       "203.0.113.7",
+			xff:        forwardedIP + ", 10.1.2.3",
+			want:       forwardedIP,
 		},
 		{
 			name:       "trusted proxy falls back to x-real-ip",
-			remoteAddr: "127.0.0.1:5000",
-			xRealIP:    "203.0.113.9",
-			want:       "203.0.113.9",
+			remoteAddr: loopbackPeer,
+			xRealIP:    realIP,
+			want:       realIP,
 		},
 		{
 			name:       "untrusted peer ignores forwarded-for",
 			remoteAddr: "198.51.100.4:5000",
-			xff:        "203.0.113.7",
+			xff:        forwardedIP,
 			want:       "198.51.100.4",
 		},
 		{
 			name:       "untrusted peer ignores x-real-ip",
 			remoteAddr: "198.51.100.4:5000",
-			xRealIP:    "203.0.113.9",
+			xRealIP:    realIP,
 			want:       "198.51.100.4",
 		},
 		{
@@ -76,7 +85,7 @@ func clientIPCases() []clientIPCase {
 		},
 		{
 			name:       "trusted proxy with garbage header uses peer",
-			remoteAddr: "127.0.0.1:5000",
+			remoteAddr: loopbackPeer,
 			xff:        "not-an-ip",
 			want:       "127.0.0.1",
 		},
@@ -119,7 +128,7 @@ func TestSecurityHeaders(t *testing.T) {
 	)
 
 	rec := httptest.NewRecorder()
-	req := httptest.NewRequest(http.MethodGet, "/", http.NoBody)
+	req := httptest.NewRequestWithContext(t.Context(), http.MethodGet, "/", http.NoBody)
 	handler.ServeHTTP(rec, req)
 
 	want := map[string]string{

backend/internal/reportbuf/reportbuf.go

@@ -163,7 +163,9 @@ func (b *Buffer) writeFile(data []byte) {
 	name := fmt.Sprintf("reports-%s.jsonl.zst", ts)
 	path := filepath.Join(b.dataDir, name)
 
-	f, err := os.OpenFile( //nolint:gosec // path built from controlled dataDir + timestamp
+	// path is built from the operator-supplied dataDir plus a
+	// generated timestamp, so it carries no external input.
+	f, err := os.OpenFile( //nolint:gosec // see comment above
 		path,
 		os.O_WRONLY|os.O_CREATE|os.O_EXCL,
 		filePerms,

backend/internal/server/server.go

@@ -62,7 +62,10 @@ func New(
 		OnStart: func(_ context.Context) error {
 			s.startupTime = time.Now().UTC()
 
-			go func() { //nolint:contextcheck // fx OnStart ctx is startup-only; run() creates its own
+			// The fx OnStart context is scoped to startup and is
+			// cancelled once the hook returns; run() derives its
+			// own context instead of inheriting this one.
+			go func() { //nolint:contextcheck // see comment above
 				s.run()
 			}()
 
@@ -94,7 +97,7 @@ func (s *Server) run() {
 }
 
 func (s *Server) serve() int {
-	var ctx context.Context //nolint:wsl // ctx must be declared before multi-assign
+	var ctx context.Context
 
 	ctx, s.cancelFunc = context.WithCancel(
 		context.Background(),