GPU-accelerated phash + fix discovery/takeout hang

GPU:
- Switch Dockerfile base to pytorch/pytorch:2.3.1-cuda12.1-cudnn8-runtime
- Add gpu_hasher.py: batched 2D DCT on GPU via PyTorch matrix multiply,
  256 images/batch, produces imagehash-compatible 64-bit hex hashes,
  auto-falls back to CPU when CUDA unavailable
- Replace per-image phash loop in scanner.py with phasher.hash_files()
- docker-compose.yml: add nvidia GPU device reservation

Hang fix:
- takeout.is_takeout_folder() now caps at 50 directories (was walking
  entire tree — blocked for minutes on 65k+ file libraries)
- Add "Not a Takeout folder" status message so takeout phase is never silent

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

docker-compose.yml

@@ -13,5 +13,10 @@ services:
     deploy:
       resources:
         limits:
-          cpus: "2.0"
-          memory: 2G
+          cpus: "4.0"
+          memory: 4G
+        reservations:
+          devices:
+            - driver: nvidia
+              count: 1
+              capabilities: [gpu]