fix(tenant): use BUNKER_RELAYS env var for default relay

[irab]

• Add get_default_relay() to read first non-empty relay from BUNKER_RELAYS
• Fix relay parsing to trim all entries before selecting first non-empty one
• Fix get_default_settings() fallback to include email_from field
• Add tests for env var relay configuration
• Fall back to wss://relay.damus.io when BUNKER_RELAYS is unset

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

---

User seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[dcadenas]

LGTM!