fix: Remove Cargo.lock from Dockerfile copy to avoid missing file error

backend/docker/Dockerfile.dev

@@ -11,7 +11,7 @@ RUN apt-get update && apt-get install -y \
     && rm -rf /var/lib/apt/lists/*
 
 # Copy Cargo files first for better caching
-COPY Cargo.toml Cargo.lock ./
+COPY Cargo.toml ./
 
 # Create dummy main.rs for dependency caching
 RUN mkdir src && echo 'fn main() {}' > src/main.rs