fix: correct torch_dtype kwarg, help text default, and CLI backend choices

[fix2015]

Summary

• serve/src/pixelrag_serve/api.py: Fixed dtype=dtype → torch_dtype=dtype in Qwen3VLForConditionalGeneration.from_pretrained() call, matching every other call site in the codebase.
• index/src/pixelrag_index/monitor.py: Fixed --validate-max-fail-pct help string that incorrectly stated (default: 10.0) when the actual default is 5.0.
• render/src/pixelrag_render/render.py: Removed "playwright" from --backend argparse choices since only the CDP backend is implemented.

Test plan

• Verified torch_dtype is the correct kwarg used by all other from_pretrained calls in the codebase
• Confirmed default=5.0 matches the corrected help text
• Confirmed no playwright backend implementation exists in the render module

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