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>

scripts/seed_remotion_templates.py

@@ -67,6 +67,10 @@ TEXTCOLORS = {
     "AppShowcase3D": "#0f172a",
 }
 
+# Templates that ship a distinct preview video PER aspect (so the detail page shows
+# the matching render, not the 16:9 cropped). Others reuse the single 16:9 preview.
+PERASPECT_VIDEO = {"AppShowcase3D"}
+
 def swatch_svg(colors):
     rects = "".join(f'<rect x="{i*50}" y="0" width="50" height="40" fill="{c}"/>' for i, c in enumerate(colors))
     return f'<svg xmlns="http://www.w3.org/2000/svg" width="200" height="40">{rects}</svg>'
@@ -94,10 +98,11 @@ for idx, (tid, slug, name, desc, dur, texts, (accent, sec, bg)) in enumerate(T):
         pid = uid(f"p-{tid}-{asp}")
         sid = uid(f"s-{tid}-{asp}")
         thumb = f"{MINIO}/template-media/{tid}-{asp}.png"
+        pvideo = f"{MINIO}/template-media/{tid}-{asp}.mp4" if tid in PERASPECT_VIDEO else preview
         out.append(
             "INSERT INTO content.projects (id,container_id,name,image,full_demo,original_width,original_height,aspect,"
             "project_duration_sec,free_fps,choose_mode,resolution,render_engine,render_remotion_comp,is_published,sort) VALUES ("
-            f"{q(pid)},{q(cid)},{q(aspstr)},{q(thumb)},{q(preview)},{w},{h},{q(aspstr)},"
+            f"{q(pid)},{q(cid)},{q(aspstr)},{q(thumb)},{q(pvideo)},{w},{h},{q(aspstr)},"
             f"{dur},30,'FLEXIBLE','FullHD','Remotion',{q(tid+'-'+asp)},TRUE,0);")
         out.append(
             "INSERT INTO content.scenes (id,project_id,key,title,scene_color_svg,default_duration_sec,sort) VALUES ("