fix: support proxy-safe subscriptions delete url

[Priveetee]

Summary

• add a proxy-safe delete path for subscriptions via DELETE /subscriptions?url=...
• make DELETE /subscriptions/{channelUrl...} resolve url from query first, then robustly decode path tail
• keep existing behavior while avoiding fragile segment reconstruction of encoded channel URLs
• add integration tests covering query delete and proxy-decoded path fallback

Why

Behind nginx, encoded subscription URLs can be decoded/normalized in a way that breaks https:// path reconstruction and causes false 404 on unsubscribe.

Validation

• full suite: ./gradlew test -> success
• new tests: SubscriptionsDeleteProxyRoutesTest