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

Re-audit fixes: typing-indicator regression test, Deno proxy tests in CI

Merge pull request #2 from S1lverElixir/dependabot/github_actions/actions/setup-python-7

Merge pull request #3 from S1lverElixir/dependabot/github_actions/actions/checkout-7

Move tests to tests/ and proxy to proxy/, update CI and docs paths

Bump actions/checkout from 6 to 7

Bump actions/setup-python from 6 to 7

P0 batch 2: diag budget, HTML truncation at source, TTL caps, tokenless register_link, backup 14d, CI permissions (AUD-J-002, AUD-F-001, AUD-G-001, AUD-D-001, AUD-D-007, AUD-M-001)

Add files via upload

Add files via upload

Add CI workflow for testing and dependency checks