fix: add cache-bust build arg to force Docker rebuild on deploy

Portainer stack updates were serving stale cached Docker layers.
Added CACHE_BUST ARG so each deploy can force a fresh build.
Set CACHE_BUST env var in Portainer to current timestamp to trigger.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Dockerfile

@@ -7,13 +7,18 @@ COPY frontend/package.json frontend/
 COPY server/package.json server/
 RUN pnpm install --frozen-lockfile
 
+# Cache-bust arg — pass --build-arg CACHE_BUST=$(date +%s) to force rebuild
+ARG CACHE_BUST=0
+
 # Stage 2: Build frontend
 FROM deps AS build-fe
+ARG CACHE_BUST
 COPY frontend/ frontend/
 RUN pnpm --filter frontend build
 
 # Stage 3: Build server
 FROM deps AS build-be
+ARG CACHE_BUST
 COPY server/ server/
 RUN pnpm --filter server build