fix: make model integrity downloads reliable

[Mr-Sunglasses]

Problem

Multi-file model installs were not a safe transaction: after one worker failed, cleanup could run while other thread-backed downloads were still writing and recreate a partial model directory. Repository-wide request bursts and silent short responses made the integrity path unnecessarily flaky. Pin harvesting also refreshed unrelated models and could combine a new revision with a stale checksum.

Summary

• Bound folder downloads to four concurrent files, cancel peers on failure, and wait for every worker to stop before deleting staging data.
• Detect short responses from Content-Length and retry network or checksum failures against the exact same pinned digest; persistent mismatches still fail and leave no file behind.
• Make pin harvesting incremental by default, replace revision plus digests atomically, preserve the previous snapshot on incomplete harvests, and fail clearly.
• Require complete pins for every Hugging Face catalog entry and document the simplified add/refresh workflow.

Verification

• just test — ruff, WPS, mypy, locked package/audit checks, 550 tests, and Compose config passed
• uv run scripts/verify-model-pins.py — 60 checked, 0 mismatched, 0 skipped
• Real production-path install of whisper.cpp:ggml-apex-hinglish-q5_0.bin — completed at 574,041,195 bytes with the committed SHA-256 pin
• uv run scripts/harvest-model-pins.py — 0 missing pinnable models and no pin-file diff
• Container build — not applicable; no Dockerfile or lockfile changed

Privacy and security

• No secrets, recordings, transcripts, or private hostnames added
• No bearer-auth, upload-limit, retention, or bind-default changes
• Digest enforcement remains fail-closed; retries never change or bypass the expected pin

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