fix: harden bounty notification key exchange retries

[kongwen686]

Summary

• Treat nil, empty, and whitespace-only V2 contact_key values as unavailable so notifications remain in the key-exchange retry path.
• Add non-2xx handling for contact lookup, add_contact, and /send calls, and close the add_contact response body.
• Preserve explicit notification statuses when saving queued notifications while still defaulting new notifications to PENDING.
• Add focused handler tests for contact-key availability, contact lookup errors, and V2 send status handling.

Why

The notification lifecycle in #2386 depends on only sending once the v2 bot has a usable contact key. The previous code only checked for nil, so an empty contact_key could be treated as ready and skip the intended WAITING_KEY_EXCHANGE retry behavior.

Validation

• git diff --check
• go test ./handlers ./db was not run because this environment does not have the Go toolchain installed (go: command not found).

Payment/contact

For bounty/payment coordination: chaoqiang.tian@gmail.com. If non-Stripe payout is needed, PayPal is hooqee@126.com; EVM is 0x225b4bC64E051827c069312f82fA590Ec9632209.

Refs #2386

[kongwen686]

Payment / contact note if this PR is accepted for a bounty or reward:

I am based in mainland China and Stripe onboarding may not be available for me. PayPal would be the preferred payout method if supported: hooqee@126.com.

Contact email: chaoqiang.tian@gmail.com
EVM address: 0x225b4bC64E051827c069312f82fA590Ec9632209
Stellar address: GDYPL56JCCHD75ZB2I5Z4WEWZFRD24LEC62XQ2MBL34653XKO65LA
RTC/ClawRTC address: RTC02d8536941d636aee4012596afbcd8185e9f8283
Alipay: chaoqiang457963@126.com

Please let me know which payout route is supported.