ci: multi-stage Dockerfile + fix workflow to match runner pattern

- Add tester/builder/runner stages to Dockerfile
- Use docker/build-push-action@v6 (matches other repos)
- Run tests via Docker tester stage (no setup-node needed)
- Fix secret names: REGISTRY_USERNAME / REGISTRY_TOKEN
- Runner target: keep yt-dlp + Chromium, copy only build artifacts

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Dockerfile

@@ -1,4 +1,21 @@
-FROM node:24-alpine
+# ── stage: tester ────────────────────────────────────────────────────────────
+FROM node:24-alpine AS tester
+WORKDIR /app
+COPY package*.json ./
+RUN npm ci
+COPY . .
+RUN npm test
+
+# ── stage: builder ───────────────────────────────────────────────────────────
+FROM node:24-alpine AS builder
+WORKDIR /app
+COPY package*.json ./
+RUN npm ci
+COPY . .
+RUN npm run build
+
+# ── stage: runner ────────────────────────────────────────────────────────────
+FROM node:24-alpine AS runner
 WORKDIR /app
 
 # Install yt-dlp (primary Instagram extractor) and Playwright system dependencies (fallback)
@@ -12,10 +29,8 @@ RUN apk add --no-cache \
     pip3 install --break-system-packages yt-dlp
 
 COPY package*.json ./
-RUN npm ci
-
-COPY . .
-RUN npm run build
+RUN npm ci --omit=dev
+COPY --from=builder /app/build ./build
 
 EXPOSE 3000
 ENV NODE_ENV=production