fix: remove unused Deno from backend Dockerfile (#34)

Deno installed in builder stage but never used. Removes ~75MB from
image and eliminates unused runtime ENV vars.

Closes #34

docker/Dockerfile.backend

@@ -1,7 +1,6 @@
 FROM python:3.12-slim AS builder
 
 RUN apt-get update && apt-get install -y --no-install-recommends curl unzip \
-    && curl -fsSL https://deno.land/install.sh | sh \
     && rm -rf /var/lib/apt/lists/*
 
 WORKDIR /build
@@ -44,9 +43,6 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
 WORKDIR /app
 COPY src/ ./src/
 
-ENV DENO_INSTALL="/root/.deno"
-ENV PATH="${DENO_INSTALL}/bin:${PATH}"
-
 ENV PLAYWRIGHT_BROWSERS_PATH=/app/.cache/ms-playwright
 RUN chown -R appuser:appuser /app