fix(deps): resolve proto before code generation

Dependabot runs go mod tidy without generating backend RPC types.
Keep a tracked package declaration so it resolves the local package
instead of searching for a nonexistent external module.

Assisted-by: Codex:gpt-6

pkg/grpc/proto/doc.go

@@ -0,0 +1,6 @@
+// Package proto contains the generated backend RPC types.
+//
+// Keep this declaration in source control so go mod tidy can resolve the local
+// package before make protogen-go generates its implementation. Dependabot runs
+// module discovery without running code generation.
+package proto