fix(detail+docker): per-aspect template preview + Debian frontend base

- Template detail page now shows the render matching the SELECTED aspect (poster +
  preview video) instead of the 16:9 cover cropped into a 9:16/1:1 box. TemplateVariant
  carries per-aspect image/previewVideo; fetchTemplateVariants + the detail page wire them.
- AppShowcase3D ships a distinct preview video per aspect (seed PERASPECT_VIDEO).
- Frontend Dockerfile: Alpine -> node:20-slim (glibc). Fixes next-swc ("ld-linux..."
  load failure that broke `next build` once libc6-compat was removed) AND the original
  CI Alpine-CDN issue. Healthcheck switched to node (slim has no wget).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Dockerfile

@@ -1,9 +1,8 @@
 # ── Stage 1: install dependencies ────────────────────────────────────────────
-FROM mirror.soroushasadi.com/node:20-alpine AS deps
-# NOTE: do NOT `apk add libc6-compat` here — the deps stage only runs `npm ci`
-# (which doesn't need it) and the build/runtime stages omit it anyway. Pulling it
-# reaches Alpine's public CDN (dl-cdn.alpinelinux.org), which is unreachable from
-# the CI server (only the Nexus mirror is) and fails the whole build.
+FROM mirror.soroushasadi.com/node:20-slim AS deps
+# Debian (glibc) base on purpose: Alpine (musl) needs `libc6-compat` for next-swc,
+# which is only on the geo-blocked Alpine CDN (unreachable from the CI server).
+# Debian ships glibc, so next-swc's gnu binary loads natively — no apk, no CDN.
 WORKDIR /app
 
 COPY package.json package-lock.json* ./
@@ -21,7 +20,7 @@ RUN for i in 1 2 3 4 5; do \
     echo "npm ci failed after 5 attempts" && exit 1
 
 # ── Stage 2: build ───────────────────────────────────────────────────────────
-FROM mirror.soroushasadi.com/node:20-alpine AS builder
+FROM mirror.soroushasadi.com/node:20-slim AS builder
 WORKDIR /app
 
 COPY --from=deps /app/node_modules ./node_modules
@@ -54,15 +53,15 @@ ENV NODE_ENV=production
 RUN npm run build
 
 # ── Stage 3: production runner ────────────────────────────────────────────────
-FROM mirror.soroushasadi.com/node:20-alpine AS runner
+FROM mirror.soroushasadi.com/node:20-slim AS runner
 WORKDIR /app
 
 ENV NODE_ENV=production
 ENV NEXT_TELEMETRY_DISABLED=1
 
-# Create a non-root user (security best practice)
-RUN addgroup --system --gid 1001 nodejs \
- && adduser  --system --uid 1001 nextjs
+# Create a non-root user (security best practice). Debian uses groupadd/useradd.
+RUN groupadd --system --gid 1001 nodejs \
+ && useradd  --system --uid 1001 --gid nodejs nextjs
 
 # Copy public assets
 COPY --from=builder /app/public ./public