(Caught one such uniqueness exception in the wild)
* 200 OK -> 204 No Content, default status
* No need to touch the subscription when found
* Drop superflous test
Add SSRF protection for web push endpoints:
- Resolve endpoint IP once and pin it for connection
- Validate endpoints resolve to public IPs
- Whitelist permitted push service hosts
Add missing IP ranges to SsrfProtection:
- 100.64.0.0/10 (Carrier-grade NAT, RFC6598)
- 198.18.0.0/15 (Benchmark testing, RFC2544)
Note: link-local (169.254.0.0/16) is already covered by ip.link_local?