fix: install rust to /usr/local so CARGO_HOME persists across Docker layers

Dockerfile

@@ -10,10 +10,17 @@ RUN curl -fsSL https://deb.nodesource.com/setup_20.x | bash - \
     && apt-get install -y nodejs \
     && rm -rf /var/lib/apt/lists/*
 
-# 3. Install Rust (stable toolchain) for Maturin compilation
-RUN curl https://sh.rustup.rs -sSf | sh -s -- -y --default-toolchain stable \
-    && /root/.cargo/bin/rustup default stable
-ENV PATH="/root/.cargo/bin:${PATH}"
+# 3. Install Rust stable - set explicit home dirs so they persist across layers
+ENV RUSTUP_HOME=/usr/local/rustup \
+    CARGO_HOME=/usr/local/cargo \
+    PATH=/usr/local/cargo/bin:${PATH}
+
+RUN curl https://sh.rustup.rs -sSf | sh -s -- -y \
+        --no-modify-path \
+        --default-toolchain stable \
+    && rustup default stable \
+    && cargo --version \
+    && rustc --version
 
 # 4. Copy backend requirements and install Python deps + maturin
 WORKDIR /app