Fix Debian install guidance, dependency floors, and vid2mp4 failures

[zackees]

Summary

• make pipx install zcmds the primary install path and document Debian/PEP 668-safe alternatives
• bump advanced-askai to the fixed 1.0.3 release and raise transcribe-anything to the Python 3.12-compatible 3.x line
• replace vid2mp4 NVENC -cq usage with -rc constqp -qp, return ffmpeg failures, and add focused regression tests
• add an imgai import smoke test for the fixed inputimeout import path

Validation

• uv run --frozen --group test ruff format --check src
• uv run --frozen --group test ruff check src
• uv run --frozen --group test pyright src
• uv run --frozen --group test python -m unittest discover tests -v
• uv run --no-project --python 3.12 --refresh-package advanced-askai --with advanced-askai==1.0.3 python -c "import importlib.metadata as m; import advanced_askai; print(m.version('advanced-askai')); print(advanced_askai.__file__)"
• uv run --no-project --python 3.12 --with transcribe-anything==3.2.10 python -c "import importlib.metadata as m; import transcribe_anything; print(m.version('transcribe-anything')); print(transcribe_anything.__file__)"

Closes #4
Closes #5
Closes #7
Closes #8
Closes #10