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

internal/handlers/handlers.go

@@ -111,6 +111,8 @@ func (s *Handlers) initImageService() error {
 		fetcherCfg.MaxConnectionsPerHost = s.config.UpstreamConnectionsPerHost
 	}
 
+	fetcherCfg.BlockedNetworks = s.config.BlockedNetworks
+
 	// Create the service
 	svc, err := imgcache.NewService(&imgcache.ServiceConfig{
 		Cache:         cache,