ctcloud: CertSpotter-backed CT cloud-resource monitor

Single stateless Go process that consumes CT logs via embedded (patched)
CertSpotter internals, classifies cloud/database/storage DNS identifiers
in-process with a zero-allocation classifier, enriches via round-robin DNS
resolvers with CNAME following, and batch-writes useful findings to PostgreSQL.

- third_party/certspotter: 2-hunk IdentifierSink patch, no stdout/JSON/files
- internal/classify: AWS/Azure/GCP/Firebase/Cloudflare R2/DigitalOcean/Supabase/OCI
- internal/dns: sharded bounded TTL cache, multi-resolver load balancing + failover
- internal/pipeline: bounded queues, dedup, direct-priority load shedding
- internal/store: pgx batch writer, Postgres CT checkpoints (no cert contents)
- internal/probe: optional non-destructive reachability checks (off by default)
- Docker: distroless non-root, read-only rootfs, no app volume, resource limits
- tests enforce 0 allocs/op on the classifier and sink hot path

third_party/certspotter/monitor/mailutils.go

@@ -0,0 +1,34 @@
+// Copyright (C) 2023 Opsmate, Inc.
+//
+// This Source Code Form is subject to the terms of the Mozilla
+// Public License, v. 2.0. If a copy of the MPL was not distributed
+// with this file, You can obtain one at http://mozilla.org/MPL/2.0/.
+//
+// This software is distributed WITHOUT A WARRANTY OF ANY KIND.
+// See the Mozilla Public License for details.
+
+package monitor
+
+import (
+	"crypto/rand"
+	"encoding/hex"
+	"os"
+)
+
+const mailDateFormat = "Mon, 2 Jan 2006 15:04:05 -0700"
+
+func generateMessageID() string {
+	var randomBytes [16]byte
+	if _, err := rand.Read(randomBytes[:]); err != nil {
+		panic(err)
+	}
+	return hex.EncodeToString(randomBytes[:]) + "@selfhosted.certspotter.org"
+}
+
+func sendmailPath() string {
+	if envVar := os.Getenv("SENDMAIL_PATH"); envVar != "" {
+		return envVar
+	} else {
+		return "/usr/sbin/sendmail"
+	}
+}