chore: archive code-port plan, add plans/todo.md index

Code port (commit bff1d32) is done; move plan to plans/archive/.
New plans/todo.md is the forward-looking index pointing at the active deploy
plan, pre-flight checks, hard gates, and open questions for the operator.

plans/todo.md

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+# Outstanding Work
+
+Quick index of what's left after the Worker code port (commit `bff1d32`).
+
+## Active plan
+
+**[260426-2327-cloudflare-deploy-and-smoke](260426-2327-cloudflare-deploy-and-smoke/plan.md)** — operator-driven; provision Atlas, set CF secrets, run hard gates, deploy, register webhook, smoke-test, write deployment docs.
+
+## Pre-flight (operator)
+
+Before running the deploy plan:
+
+- [ ] `npm install` (first time only — pulls `mongodb` + `wrangler`)
+- [ ] MongoDB Atlas account created (free, no credit card)
+- [ ] Cloudflare account created (free Workers plan)
+- [ ] `npx wrangler login` complete
+
+## Hard gates that can abort the deploy plan
+
+| Gate | Threshold | Pivot if fail |
+|---|---|---|
+| Bundle size (`wrangler deploy --dry-run`) | ≤ 2.7 MiB (10% headroom under 3 MiB Free cap) | Switch storage to Upstash Redis (no driver, HTTP-only) |
+| Cold-start CPU (`/__mongo-ping`) | < 40ms (10ms headroom under 50ms Free cap) | Workers Paid ($5/mo) OR pivot to Upstash |
+| Atlas auto-pause | Catchable error within 5s, not a hang | Document catch path; not abort-worthy |
+
+`mongodb` driver is the dominant risk — bundles ~4–5 MiB compressed against the 3 MiB Free cap. miti99bot has the same risk and has not yet validated the gate end-to-end on real CF Free either.
+
+## Open questions for the operator
+
+1. Atlas account: existing or first-time setup?
+2. Bot username for `setMyCommands`: same as `.env.example`?
+3. Greenfield Mongo data, or existing data to import? (Existing → schedule a separate import phase.)
+4. Custom domain or `*.workers.dev` URL? (Webhook works on either.)
+
+## Backlog (post-deploy, out of current scope)
+
+- [ ] Tests (none exist; original Node port has none either)
+- [ ] Quarterly Atlas password rotation reminder
+- [ ] Observability dashboard (CF Workers + Atlas charts)
+- [ ] CI workflow on push (lint + dry-run bundle size as PR check)
+- [ ] Telegram bot description / about-text via `setMyDescription` (post-deploy)
+
+## Reference
+
+- **miti99bot** validated the same stack (CF Workers + Atlas + `nodejs_compat_v2` + `mongodb`): `/config/workspace/tiennm99/miti99bot/plans/260425-1945-mongodb-atlas-migration/`
+- **Archived code-port plan** (history of what's now done): [archive/260426-2015-cloudflare-worker-code-port](archive/260426-2015-cloudflare-worker-code-port/plan.md)