refactor: rename auth/firebase.go to token_verifier.go

The file no longer contains Firebase-specific code — only the TokenVerifier
interface. Rename to reflect its actual purpose.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/internal/auth/token_verifier.go

@@ -0,0 +1,8 @@
+package auth
+
+import "context"
+
+// TokenVerifier abstracts token verification for testability.
+type TokenVerifier interface {
+	VerifyToken(ctx context.Context, idToken string) (uid, email, name, avatarURL string, err error)
+}