Medium wave 4: validated env numbers, Upstash backend and shutdown flush tests, deno check/lint in CI, reproducible pip step, docs for missing variables

Stale text: drop dead /model ref from prompt, fix test counts and TTS order notes

Finish audit leftovers: speed_key dedup, truncate in formatting, honest mime suffixes, filename sanitize, tiger boundary, dead image data, deterministic timing tests, HEALTHCHECK, dependabot (AUD-B-003, AUD-E-005, AUD-E-006, AUD-K-001, AUD-M-001, AUD-M-004)

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