Add Puppeteer fallback for Cloudflare-protected sites

When HTTP requests are blocked with 403 (e.g., B&H Photo's Cloudflare
protection), the scraper now automatically retries using a headless
Chrome browser via Puppeteer. Also updated Dockerfile to include
Chromium dependencies for container deployment.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/Dockerfile

@@ -1,5 +1,5 @@
 # Build stage
-FROM node:20-alpine AS builder
+FROM node:20-slim AS builder
 
 WORKDIR /app
 
@@ -16,10 +16,38 @@ COPY . .
 RUN npm run build
 
 # Production stage
-FROM node:20-alpine AS production
+FROM node:20-slim AS production
 
 WORKDIR /app
 
+# Install Chromium dependencies for Puppeteer
+RUN apt-get update && apt-get install -y \
+    chromium \
+    fonts-liberation \
+    libasound2 \
+    libatk-bridge2.0-0 \
+    libatk1.0-0 \
+    libcups2 \
+    libdbus-1-3 \
+    libdrm2 \
+    libgbm1 \
+    libgtk-3-0 \
+    libnspr4 \
+    libnss3 \
+    libx11-xcb1 \
+    libxcomposite1 \
+    libxdamage1 \
+    libxfixes3 \
+    libxrandr2 \
+    libxss1 \
+    xdg-utils \
+    --no-install-recommends \
+    && rm -rf /var/lib/apt/lists/*
+
+# Set Puppeteer to use installed Chromium
+ENV PUPPETEER_SKIP_CHROMIUM_DOWNLOAD=true
+ENV PUPPETEER_EXECUTABLE_PATH=/usr/bin/chromium
+
 # Copy package files
 COPY package*.json ./
 
@@ -30,8 +58,10 @@ RUN npm install --omit=dev
 COPY --from=builder /app/dist ./dist
 
 # Create non-root user
-RUN addgroup -g 1001 -S nodejs && \
-    adduser -S nodejs -u 1001
+RUN groupadd -r nodejs && useradd -r -g nodejs nodejs
+
+# Change ownership of app directory
+RUN chown -R nodejs:nodejs /app
 
 USER nodejs