Warn at startup when PUBLIC_HOST/DB connection env vars are missing

[birme]

Summary

• Adds startup Log().warn(...) warnings when PUBLIC_HOST is not set (falls back to localhost default) and when neither DB_CONNECTION_STRING nor MONGODB_CONNECTION_STRING is set (uses localhost MongoDB default).
• Matches the style/placement of the existing SMB_ADDRESS startup check. Minimal addition only; no restructuring.

Test plan

• npm run typecheck clean
• npm test — all 243 tests pass ("worker process has failed to exit gracefully" warning is expected/pre-existing)
• npm run lint clean on changed file

Closes #268

[birme]

code-reviewer verdict: Needs Changes

Code Review

Summary: PR #301 (#268) adds two startup Log().warn(...) calls for missing PUBLIC_HOST and missing DB_CONNECTION_STRING/MONGODB_CONNECTION_STRING. Conditions correctly mirror the existing ?? fallbacks and don't break startup. Style clean, no npm-hygiene issues. Held back on required test coverage + a message-accuracy nit.

Blocking: None.

Warnings:

• src/server.ts (no test) — Criteria require a unit test covering missing-vs-present env branches (mocking ./log). No server.test.ts exists/was added.
• src/server.ts (testability) — The warning logic is top-level in a file whose startServer() IIFE runs on import (and connects to the DB), so it's not unit-testable as written. Extract into an exported pure warnMissingEnv(env, log).
• src/server.ts:23-25 — Message names only DB_CONNECTION_STRING though the guard also depends on MONGODB_CONNECTION_STRING; name both to avoid operator confusion.

3 Warnings with no Blocking → Needs Changes. Moving back to Ready to add the extracted helper + test.