revert(deps): keep rembg at 2.0.69 (2.0.75 conflicts with pinned numpy==1.26.4)

rembg 2.0.75 requires a numpy incompatible with the pinned numpy==1.26.4
that the rest of the ML stack (onnxruntime etc.) depends on, making
pip-audit's resolution impossible. The rembg <2.0.75 advisory (medium) is
accepted as a residual: it only affects the on-demand background-removal AI
bundle (publishing currently paused) and can't be patched without a numpy
2.x migration across the whole Python sidecar.

packages/ai/python/requirements-gpu.txt

@@ -1,4 +1,4 @@
-rembg==2.0.75
+rembg==2.0.69
 realesrgan==0.3.0
 paddleocr==2.9.1
 paddlepaddle-gpu==3.0.0

packages/ai/python/requirements.txt

@@ -1,4 +1,4 @@
-rembg[cpu]==2.0.75
+rembg[cpu]==2.0.69
 realesrgan==0.3.0
 paddleocr[doc-parser]>=3.4.0,<3.5.0
 paddlepaddle>=3.0.0,<3.1.0