Better errors for when secrets (POLICY_AUD, etc) are not set

Depending on what isn't set, you get some mysterious errors that takes a while to figure out by trawling the server logs.

- `TEAM_DOMAIN` not set -> `401 Authentication error`
- `POLICY_AUD` not set -> 500 error / `[Auth] JWT validation failed: unexpected "aud" claim value`
- `ACCOUNT_ID` or `API_KEY` not set -> `API error: 500` / `Error: Failed to list databases: 404`

Why not check if these are set before doing anything and show an error for each? e.g.

`Error: TEAM_DOMAIN env/secret not set`

Bonus: As well as the error, link to docs such as: https://developers.cloudflare.com/fundamentals/account/find-account-and-zone-ids/

No files or tests are named. Start by tracing where TEAM_DOMAIN, POLICY_AUD, ACCOUNT_ID, and API_KEY are read before authentication and database/API requests; done means each missing secret produces a clear named error and includes the relevant documentation link.

Written by the indexing model from the issue text.