feat: store layer with indexes, optimistic concurrency, GridFS, credential crypto (phase 2)

- mongo-driver v2 connection with bounded startup retry + /readyz gate
- idempotent creation of all §4.9 indexes
- UpdateVersioned: version-filtered updates, conflict vs not-found errors
- AES-256-GCM Seal/Open for ticketing credentials, base64(nonce|ct)
- GridFS file store: MIME sniffing, MAX_UPLOAD_MB cap, sha256 metadata
- HMAC-signed short-lived file URL tokens (§5.1, 1h TTL)
- integration tests against compose Mongo via docker-compose.test.yml overlay

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

internal/files/sign_test.go

@@ -0,0 +1,62 @@
+package files
+
+import (
+	"strings"
+	"testing"
+	"time"
+)
+
+func TestSignVerifyRoundTrip(t *testing.T) {
+	secret := []byte("0123456789abcdef0123456789abcdef")
+	now := time.Now()
+	token := SignToken(secret, "01JFILE", now.Add(time.Hour))
+
+	if !VerifyToken(secret, "01JFILE", token, now) {
+		t.Fatal("fresh token must verify")
+	}
+	if !VerifyToken(secret, "01JFILE", token, now.Add(59*time.Minute)) {
+		t.Fatal("token must verify just before expiry")
+	}
+}
+
+func TestVerifyRejections(t *testing.T) {
+	secret := []byte("0123456789abcdef0123456789abcdef")
+	now := time.Now()
+	token := SignToken(secret, "01JFILE", now.Add(time.Hour))
+
+	tests := []struct {
+		name   string
+		fileID string
+		token  string
+		at     time.Time
+	}{
+		{"expired", "01JFILE", token, now.Add(2 * time.Hour)},
+		{"wrong file", "01JOTHER", token, now},
+		{"garbage token", "01JFILE", "not-a-token", now},
+		{"empty token", "01JFILE", "", now},
+		{"truncated token", "01JFILE", token[:len(token)-10], now},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if VerifyToken(secret, tt.fileID, tt.token, tt.at) {
+				t.Error("must not verify")
+			}
+		})
+	}
+
+	if VerifyToken([]byte("another-secret-another-secret-xx"), "01JFILE", token, now) {
+		t.Error("different secret must not verify")
+	}
+}
+
+func TestSignedURLShape(t *testing.T) {
+	secret := []byte("0123456789abcdef0123456789abcdef")
+	u := SignedURL("http://localhost:8787", secret, "01JFILE", time.Now().Add(time.Hour))
+	if !strings.HasPrefix(u, "http://localhost:8787/files/01JFILE?st=") {
+		t.Fatalf("unexpected URL %q", u)
+	}
+	token := u[strings.Index(u, "?st=")+4:]
+	if !VerifyToken(secret, "01JFILE", token, time.Now()) {
+		t.Fatal("token embedded in URL must verify")
+	}
+}