Fix stale UI cache, BEN2 edge quality, and fresh-install permissions

- Serve /dist and /src with Cache-Control: no-cache so browsers always
  revalidate the webpack bundle (it has a fixed filename with no content
  hash, so a stale browser cache was hiding the new Remove Background
  model dropdown after the rebuild).
- Enable BEN2's refine_foreground pass by default - it's the model's own
  documented option for preserving fine/semi-transparent edge detail
  (text borders, light rays) instead of a harder cutout, at a small
  speed cost.
- Make install-local-gpu.sh and bring-up-local-gpu.sh pre-create ./data
  as the invoking non-root user before Docker ever runs, so its daemon
  never gets a chance to auto-create those bind-mount dirs as root.
- Make prefetch-models.sh self-heal a root-owned ./data automatically
  (sudo chown) instead of erroring out with a manual fix-it command.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ro4PwQKvSc3CH19LSN21Ht

backend/app/routers/tools.py

@@ -369,7 +369,10 @@ async def _remove_background_ben2(img: Image.Image) -> bytes:
         _ben2_model.to(device).eval()
         print("BEN2_Base model loaded")
 
-    result = _ben2_model.inference(img.convert('RGB'), refine_foreground=False)
+    # refine_foreground=True runs BEN2's extra foreground-color refinement pass
+    # (slower, but recovers fine/semi-transparent edge detail instead of a hard
+    # cutout — matters for things like lace, light rays, or fine text borders).
+    result = _ben2_model.inference(img.convert('RGB'), refine_foreground=True)
 
     buffer = BytesIO()
     result.save(buffer, format='PNG')