v0.3.1: Fix opencv conflict, graceful GPU fallback, correct docs

- Remove opencv-python from [gpu] extra (conflicts with headless in base deps)
- Add graceful fallback in 'invisible' and 'all' commands when GPU deps missing
- Cache InvisibleEngine in batch mode (avoid reloading model per image)
- Fix --humanize help text (was '0.0-1.0', actual range is 0-6.0+)
- Fix stale docstring referencing non-existent [invisible] extra
- Add [gpu] extra install instructions to README
- Fix broken NeuralBleach placeholder URL in Credits

pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "remove-ai-watermarks"
-version = "0.3.0"
+version = "0.3.1"
 description = "Remove visible and invisible AI watermarks from images (Gemini / Nano Banana, ChatGPT, Stable Diffusion)"
 readme = "README.md"
 requires-python = ">=3.10"
@@ -26,7 +26,6 @@ gpu = [
     "controlnet-aux>=0.0.9",
     "safetensors",
     "ultralytics>=8.0.0",
-    "opencv-python>=4.8.0",
 ]
 dev = [
     "pytest>=8.0.0",