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.

scripts/deploy.sh

@@ -0,0 +1,94 @@
+#!/usr/bin/env bash
+# Zero-downtime redeploy:
+#   build → blue/green streamer swap → recreate API.
+# Skips everything if the rebuilt image hasn't changed (FORCE=1 to override).
+set -euo pipefail
+cd "$(dirname "$0")/.."
+
+export DOCKER_BUILDKIT=1
+export COMPOSE_DOCKER_CLI_BUILD=1
+
+IMAGE="${IMAGE:-tdurieux/anonymous_github:v2}"
+REPLICAS="${STREAMER_REPLICAS:-4}"
+TIMEOUT="${HEALTH_TIMEOUT:-90}"
+FORCE="${FORCE:-0}"
+
+log()  { printf '==> %s\n' "$*"; }
+warn() { printf '!!  %s\n' "$*" >&2; }
+
+image_id() { docker image inspect --format '{{.Id}}' "$1" 2>/dev/null || true; }
+
+wait_healthy() {
+    local name="$1" waited=0 status
+    while (( waited < TIMEOUT )); do
+        status="$(docker inspect -f \
+            '{{if .State.Health}}{{.State.Health.Status}}{{else if .State.Running}}running{{else}}stopped{{end}}' \
+            "$name" 2>/dev/null || echo missing)"
+        [[ $status == healthy || $status == running ]] && return 0
+        sleep 2; waited=$((waited + 2))
+    done
+    warn "$name not healthy after ${TIMEOUT}s (status=$status)"
+    return 1
+}
+
+list_streamers() {
+    docker compose ps --status running --format '{{.Name}}' streamer 2>/dev/null
+}
+
+# 1. Build
+log "Building image"
+before="$(image_id "$IMAGE")"
+docker compose build anonymous_github
+after="$(image_id "$IMAGE")"
+
+# 2. Cold start
+mapfile -t olds < <(list_streamers)
+if [[ ${#olds[@]} -eq 0 ]]; then
+    log "Cold start — bringing the whole stack up"
+    docker compose up -d --remove-orphans
+    docker compose ps
+    exit 0
+fi
+
+# 3. No-op if image unchanged
+if [[ -n $before && $before == "$after" && $FORCE != 1 ]]; then
+    log "Image unchanged — nothing to deploy (FORCE=1 to override)"
+    docker compose ps
+    exit 0
+fi
+
+# 4. Blue/green streamer swap
+log "Swapping streamer (${#olds[@]} old → ${REPLICAS} new)"
+docker compose up -d --no-deps --no-recreate \
+    --scale "streamer=$((${#olds[@]} + REPLICAS))" streamer >/dev/null
+
+# Anything not in $olds is new.
+declare -A was_old=()
+for o in "${olds[@]}"; do was_old[$o]=1; done
+news=()
+while IFS= read -r n; do
+    [[ -n $n && -z ${was_old[$n]:-} ]] && news+=("$n")
+done < <(list_streamers)
+
+# Wait for all new replicas to be healthy in parallel.
+pids=()
+for n in "${news[@]}"; do wait_healthy "$n" & pids+=($!); done
+fail=0
+for p in "${pids[@]}"; do wait "$p" || fail=1; done
+if [[ $fail == 1 ]]; then
+    warn "new replicas unhealthy — keeping olds in place"
+    exit 1
+fi
+
+# Drop olds and reconcile scale.
+docker rm -f "${olds[@]}" >/dev/null
+docker compose up -d --no-deps --no-recreate --scale "streamer=${REPLICAS}" streamer >/dev/null
+
+# 5. Recreate API. --no-deps: we already manage every dependency above,
+#    skipping compose's own depends_on health re-poll (~5–10s).
+log "Recreating API"
+docker compose up -d --no-deps --remove-orphans anonymous_github
+api="$(docker compose ps -q anonymous_github | head -1)"
+[[ -n $api ]] && wait_healthy "$api" || true
+
+docker compose ps