fix: prevent false GPU detection when CUDA image runs without GPU

The STIRLING_GPU=true env var was baked into the :cuda Dockerfile,
which made gpu_available() return True without checking actual
hardware. On machines without a GPU, this would crash upscale.py
(torch.device("cuda") fails) and ocr.py (PaddleOCR use_gpu=True).

Fix: the env var can only disable GPU (set to false/0), never
force-enable it. Hardware detection always runs. Removed the
baked env var from the Dockerfile since it adds no value now.

docker/Dockerfile

@@ -190,8 +190,7 @@ ENV PORT=1349 \
     CONCURRENT_JOBS=3 \
     MAX_MEGAPIXELS=100 \
     RATE_LIMIT_PER_MIN=100 \
-    STIRLING_VARIANT=${VARIANT} \
-    STIRLING_GPU=${GPU}
+    STIRLING_VARIANT=${VARIANT}
 
 # Create non-root user for runtime
 RUN groupadd -r stirling && useradd -r -g stirling -d /app -s /sbin/nologin stirling