Add DNS and HTTP probes

[rkline0x]

Supplement the TCP direct probe with:

• DNS resolution probe: resolve t.me, web.telegram.org, telegram.org and compare against known-good IP ranges (149.154.160.0/20, 91.108.4.0/22, 91.108.56.0/22). Catches DNS poisoning.
• HTTP probe: GET https://web.telegram.org/ and https://t.me/ — check for block pages or unexpected responses. Catches HTTP-layer blocking that passes TCP.

Both use stdlib only (socket, http.client).