Fix post-merge Pixelbot 3.0 audit findings

[pierrebrunelle]

The merged Pixelbot 3.0 app had several post-upgrade defects that only appeared in packaging and real Pixeltable 0.7.7 catalog use. This audit fixes the broken sdist, invalid video-frame column references, pipeline metadata failures, unsafe URL/media paths, and stale agents frontend assumptions.

The service now uses the public 0.7.7 frame and iterator metadata, handles B-tree indexes with null parameters, writes slideshows directly with PyAV, restricts URL downloads and generated-media paths, rejects unsupported legacy Office formats, and caps read/scratch responses. Directly imported media/document packages are declared, CI runs pytest on Python 3.11 through 3.14, verifies fresh dependency resolution, and rejects polluted source distributions.

Validation:

• 24 backend tests; Ruff format/check; mypy
• 5 frontend tests; ESLint; production build (271.64 kB initial chunk)
• Pixeltable schema check and clean diff: 19/19 models up to date, zero operations
• Real managed service: health 200, private URL rejected, 19 pipeline nodes, 38 indexes, expected iterator labels, zero route/node errors, clean shutdown
• Static table-column audit: 13 route-bound tables, zero missing columns
• Fresh resolution selects Pixeltable 0.7.7; lock is clean
• Wheel/sdist build from source distribution and clean wheel install; sdist reduced from 4,335 files / 86 MB to 91 files / under 1 MB

Provider-backed computed calls and hosted deployment remain outside this local audit; provider tests use mocks only.