Add Colab free PoC and opt-in GPU setup

[omar4050]

Why

Provide a fully free, runnable Google Colab proof-of-concept so users can generate simple videos without paid APIs or tokens, and an opt-in path for testing local GPU-based video generation.

Approach

• Adds a zero-API Colab notebook (notebooks/colab_free_poc.ipynb) that builds a demo using Wikimedia images, gTTS narration, and moviepy/ffmpeg.
• Adds an optional GPU notebook (notebooks/colab_gpu_optional.ipynb) with guarded, opt-in cells to: check torch/CUDA, optionally install diffusers stack, set VIDEO_GEN_LOCAL_* env vars, run a safe WanVideo probe, and a commented test-generate cell with clear warnings.
• Updates COLAB_SETUP.md with step-by-step Colab setup, guidance for enabling local generation, and safe status checks.

Notes & review guidance

• Heavy model downloads are explicitly opt-in; the notebooks avoid forcing large downloads. Warn reviewers that running the opt-in test generate will download multi-GB checkpoints and may require a HuggingFace token for some models.
• The zero-API demo requires no tokens and runs on Colab GPU/CPU. The local diffusers path is untested here and provided as an opt-in workflow.
• Reviewers should confirm the instructions in COLAB_SETUP.md and that the opt-in cells are clearly commented and safe by default.

Files changed

• COLAB_SETUP.md: add Colab setup + opt-in GPU guidance
• notebooks/colab_free_poc.ipynb: zero-API demo (already present in branch)
• notebooks/colab_gpu_optional.ipynb: new opt-in GPU notebook

Testing

• Run notebooks/colab_free_poc.ipynb on Colab to verify zero-API demo.
• Run the status-check cell in notebooks/colab_gpu_optional.ipynb before any install or model download.

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