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.

.dockerignore

@@ -1,5 +1,23 @@
+.git
+.github
+.vscode
+.idea
+.claude
+node_modules
+build
 /repositories
 db_backups
-build
-node_modules
-.github
+coverage
+.nyc_output
+*.log
+.env
+.env.*
+tests
+test
+docs
+*.md
+claude-files
+scripts
+.dockerignore
+Dockerfile*
+docker-compose*.yml