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.

docker-compose.yml

@@ -1,5 +1,3 @@
-version: "3.8"
-
 services:
   anonymous_github:
     build: .
@@ -21,13 +19,17 @@ services:
         - CMD
         - node
         - healthcheck.js
-      interval: 10s
-      timeout: 10s
-      retries: 5
+      interval: 5s
+      timeout: 5s
+      retries: 3
+      start_period: 2s
     depends_on:
-      - mongodb
-      - redis
-      - streamer
+      mongodb:
+        condition: service_healthy
+      redis:
+        condition: service_healthy
+      streamer:
+        condition: service_healthy
 
   streamer:
     build: .
@@ -50,9 +52,10 @@ services:
         - CMD
         - node
         - healthcheck.js
-      interval: 10s
-      timeout: 10s
-      retries: 5
+      interval: 5s
+      timeout: 5s
+      retries: 3
+      start_period: 2s
 
   redis:
     image: "redis:alpine"