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Chunk parts > 19MB are stored to Telegram but getFile cannot resolve files over 20MB ('Bad Request: file is too big'), making every part undownloadable (prod bug 2026-08-01: 48MB chunk -> download 500).
- src/env.ts: reject TELEGRAM_CHUNK_SIZE_BYTES > 19922944 at startup (log error + throw), default changed 20MB -> 19MB
- src/shared/utils/validation.ts: TELEGRAM_CHUNK_SIZE_MAX_BYTES constant; asSafeChunkSize now enforces the max at runtime (covers S3 multipart parts too)
- test/env.test.ts: unit tests + subprocess fail-fast tests (48MB rejected, 19MB accepted)
- test/helpers/setup-env.ts: pin safe chunk size so a stale .env can't break the suite
- .env.example + CLAUDE.md: document the 20MB getFile limit