kernel: MediaProcessor adapter — Pillow for image validation/dimensions/thumbnails

[JArmandoAnaya]

Context

First concrete implementation of the MediaProcessor port for still images.

Deliverables

• Validate + decode JPEG/PNG (extend the accepted list deliberately, document it);
  corrupt/unsupported files produce structured per-file errors, never swallowed
  (v1 exporter lesson).
• Extract width/height/format; EXIF orientation handled (dimensions reported
  post-orientation; document the policy).
• Thumbnail generation (max edge configurable, default 256px) returned as bytes for the
  blob store.

Acceptance criteria

• Corrupt file test yields a structured error including the filename
• EXIF-rotated fixture reports oriented dimensions
• Thumbnails deterministic enough for content addressing (fixed encoder params)

[JArmandoAnaya]

Heads-up from #22 (PR #87, merged 8cb66ab): Pillow is already a runtime dependency. pillow>=11.0 is in [project].dependencies — #22's image fixtures needed it, and it was going to land in the same place here, so doing it once avoided a second pyproject.toml edit. Nothing else in this issue changes; just don't re-add it.

Also already available for this task, in tests/fixtures/media.py:

• write_image / write_images — tiny PNG/JPEG, deterministic (equal arguments → byte-identical bytes, which is what "thumbnails deterministic enough for content addressing" needs).
• write_exif_rotated_image — orientation 6 on a 32×24 JPEG, so the oriented-dimensions criterion has a fixture whose right answer is 24×32.
• write_corrupt_image — real header, truncated body: sniffing succeeds and decoding is what fails, which is the structured per-file error path.
• write_unsupported_file — not an image at all, the other half of that path.

Import them as from tests.fixtures.media import ... (pythonpath = ["."] is set in the pytest config).