feat: blocked_networks config and extended SSRF ranges (closes #67)

Add a blocked_networks config key: a list of CIDRs parsed with net/netip,
added to (not replacing) the built-in SSRF blocklist. An invalid CIDR
aborts startup naming the key and the offending value.

Extend the built-in blocklist to CGNAT 100.64.0.0/10, IETF protocol
assignments 192.0.0.0/24, benchmark 198.18.0.0/15, and NAT64 64:ff9b::/96,
unmapping IPv4-mapped IPv6 so the IPv4 ranges are caught in both forms.
Enforcement stays in the dial-time re-resolution (dialSSRFSafe), which now
also consults the operator-supplied prefixes, so the DNS-rebinding window
remains closed.

Model: opus-4-8

README.md

@@ -111,6 +111,10 @@ Configured via YAML file (`--config`). Key settings:
 
 - `access_control_allow_origin` — CORS origin
 - `allowlist_hosts` — list of allowed upstream hosts
+- `blocked_networks` — list of CIDR ranges to refuse for SSRF protection,
+  added to the always-enforced built-in ranges (loopback, private,
+  link-local, CGNAT, benchmark, NAT64, and the like); an invalid CIDR
+  aborts startup
 - `upstream_fetch_timeout` — timeout for origin requests
 - `upstream_max_response_size` — max origin response size
 - `downstream_timeout` — client response timeout