perf(deploy): faster builds and zero-downtime streamer rollover

- Multi-stage Dockerfile with BuildKit npm cache mounts and a separate
  prod-deps stage so source edits don't reinstall or prune.
- Tighter .dockerignore to shrink build context.
- Healthchecks: add start_period and tighten interval/retries so
  containers report healthy as soon as the process is actually ready
  instead of after a full polling interval.
- Move recoverStuckPreparing() off the startup critical path; the
  recovery sweep now runs in the background after app.listen.
- depends_on uses condition: service_healthy and the obsolete
  compose 'version' key is gone.
- New scripts/build.sh + scripts/deploy.sh: deploy.sh builds, exits
  early if the image is unchanged, runs a blue/green streamer swap
  (scale to 2N, wait healthy in parallel, drop olds), then recreates
  the API with --no-deps to avoid compose's depends_on re-poll.

Dockerfile

@@ -1,28 +1,40 @@
-# syntax=docker/dockerfile:1
-FROM node:21-slim AS builder
+# syntax=docker/dockerfile:1.7
 
+# Stage 1: install all deps (with cache mount so npm cache survives across builds)
+FROM node:21-slim AS deps
 WORKDIR /app
-
 COPY package.json package-lock.json ./
-RUN npm ci
+RUN --mount=type=cache,target=/root/.npm \
+    npm ci --prefer-offline --no-audit --fund=false
 
-COPY tsconfig.json gulpfile.js ./
+# Stage 2: build TypeScript + gulp assets. Source changes only invalidate from here.
+FROM node:21-slim AS build
+WORKDIR /app
+COPY --from=deps /app/node_modules ./node_modules
+COPY package.json package-lock.json tsconfig.json gulpfile.js ./
 COPY public ./public
 COPY src ./src
-RUN npm run build && npm prune --omit=dev && npm cache clean --force
+RUN npm run build
 
+# Stage 3: prod-only deps. Independent of source so it caches well.
+FROM node:21-slim AS prod-deps
+WORKDIR /app
+COPY package.json package-lock.json ./
+RUN --mount=type=cache,target=/root/.npm \
+    npm ci --omit=dev --prefer-offline --no-audit --fund=false && \
+    npm cache clean --force
+
+# Stage 4: minimal runtime
 FROM node:21-alpine AS runtime
-
 ENV NODE_ENV=production
 ENV PORT=5000
 EXPOSE 5000
-
 WORKDIR /app
 
-COPY --from=builder /app/node_modules ./node_modules
-COPY --from=builder /app/build ./build
-COPY --from=builder /app/public ./public
-COPY --from=builder /app/package.json ./package.json
+COPY --from=prod-deps /app/node_modules ./node_modules
+COPY --from=build /app/build ./build
+COPY --from=build /app/public ./public
+COPY package.json ./package.json
 COPY healthcheck.js ./healthcheck.js
 
 CMD ["node", "./build/server/index.js"]