Harden model downloads, output disclosure, and supply chain

[nathan-v]

Security-hardening pass on the model registry and container build, from a 7-finding review. No behavior change for trusted zoo models; the mocked suite (108 tests) passes and a full default-profile docker build succeeds.

Findings addressed

1. Unbounded model download size. registry._fetch now enforces a per-model byte cap twice: it rejects a declared Content-Length over the limit, then tracks a running total during streaming so a lying server can't overshoot (partial .part cleaned up). Added a total-cache byte cap alongside the existing count cap. New: CORALAPI_MAX_MODEL_BYTES (256 MB), CORALAPI_MAX_MODEL_CACHE_BYTES (4 GB); label sidecars get a tight 16 MB cap.
2. No format allowlist. CORALAPI_ALLOWED_IMAGE_FORMATS (opt-in) → 415 on a disallowed decoded format. Full PIL content validation still runs regardless.
3. Error leak on reset. /v1/reset failures route through errors.py instead of returning str(exc).
4. No model integrity. Optional sha256 pinning via CORALAPI_MODEL_CHECKSUMS; a mismatched download is deleted and rejected.
5. Supply chain. Dockerfile pins uv by version (was :latest) and verifies the libedgetpu .deb and any URL-sourced tflite wheel against a sha256sum build-arg, failing the build on mismatch.
6. Path disclosure. /v1/models returns filenames not absolute paths; /readyz, /v1/status, and the reset response report device counts, not sysfs paths.
7. Unbounded K8s models volume. models emptyDir gets sizeLimit: 5Gi.

Verification

• 108 tests pass; new coverage for byte caps, checksum match/mismatch, path redaction, format allowlist, and the reset error-leak.
• ruff check / ruff format --check clean.
• Full docker build of the default (avx2) profile succeeds, exercising the new checksum verification and uv pin.

Redeploy note

Rebuilding the compat profile now requires passing its libedgetpu checksum (documented in the README build-profiles table):
--build-arg LIBEDGETPU_SHA256=4ffb6c2251b61535afb6b368d821211a914d2d4783770ec7e6001c073c2d193b