feat: add SOCKS5/Tor proxy support via PROXY_URL

[deymosh]

Description

Adds support for routing all outgoing HTTP and relay connections through a SOCKS5/Tor proxy using a new configuration variable.

Changes

• config.go: Added PROXY_URL configuration parameter, mapped to the PROXY_URL environment variable. Fixes .onion string prefixing logic for relays.
• main.go: Added createPoolWithProxy to initialize a custom http.DefaultTransport using a SOCKS5 dialer when PROXY_URL is provided.
• .env.example: Documented the new PROXY_URL variable in the configuration template.
• Connectivity Check: Added a startup debug helper (testTorConnectivity) that queries the official Tor Project API to verify if traffic is successfully routing through the Tor network.

How to test

1. Set the environment variable in your .env file: PROXY_URL="127.0.0.1:9050" (or your local Tor socks port).
2. Start the application.
3. Check the startup logs to verify the ✅ Successfully verified - traffic is routing through Tor network! message and the exit node IP.

[deymosh]

The proxy setup only works with the new library fiatjaf.com/nostr@master. The necessary changes have been made, but this would drop support for BadgerDB. If this is still acceptable, the README and the .env.example will need to be updated as well.

Tested connections to import onion relays and blasting to onion relays.