Add SAM click-to-select with brush refinement in AI Edit tool

Backend:
- sam_service.py: auto-downloads SAM ViT-B (~375 MB) on first use with
  progress tracking; loads model to CUDA/MPS/CPU; predict_points() takes
  multi-point prompts (include/exclude labels) and returns best mask
- POST /api/segment/point: SAM point-prompt endpoint; returns mask PNG
- GET  /api/segment/install-status: poll download progress
- POST /api/segment/install: explicit trigger (also auto on first click)
- main.py: pre-download SAM on startup alongside NCNN

Frontend (ai_edit.js):
- Click mode (default): click object → SAM generates mask instantly
  Alt+click → subtract (deselect over-selected area)
  Multiple clicks accumulate for multi-object or refinement
- Brush + / Brush − modes: paint to add or erase from SAM mask by hand
- If SAM model is still downloading on first click: inline progress bar,
  user retries the click when done
- Unified action bar: Erase | Replace (inline prompt) | Upscale | Expand | Clear
- All modes share the same mask canvas; SAM and brush are fully composited

https://claude.ai/code/session_01B58MaJCU1R6KwBDJCp8AfN

backend/app/main.py

@@ -21,6 +21,9 @@ async def lifespan(app: FastAPI):
     caps = probe_upscale_capabilities()
     if not caps["realesrgan_pytorch"] and not caps["realesrgan_ncnn"]:
         asyncio.create_task(ensure_ncnn_installed())
+    # Pre-download SAM model in background so first click is fast
+    from app.services.sam_service import ensure_sam_installed
+    asyncio.create_task(ensure_sam_installed())
     yield