feat(v2): complete v2 reimplementation

docs/security/notifications.md

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+# Notification security policy (M12)
+
+Webhook callbacks accept absolute `http` and `https` URLs only. HTTP is an approved compatibility option, not a relaxation of egress controls. URLs with credentials, fragments, literal IP addresses, or ports outside 80/443/8080/8443 are rejected. Immediately before every request the worker resolves all A/AAAA answers; any non-global answer rejects the whole destination. The transport connects only to an approved answer, verifies the connected peer address, disables proxy environment use, preserves HTTPS SNI/certificate validation, and rejects redirects.
+
+Webhook bodies are canonical JSON event envelopes. `X-Backup-Event-ID` is stable over retries. Each attempt supplies the versioned HMAC-SHA-256 timestamp/body input and one `X-Backup-Signature` header per active or overlap key. Key IDs and monotonically increasing subscription-local versions identify keys; an old key is retained only until its configured overlap expiry. Key material is encrypted in `secrets` and is never returned, logged, exported in recovery bundles, or included in audit/event payloads.
+
+SMTP is configured as one write-only password-backed singleton. Delivery performs EHLO, verified STARTTLS, a second EHLO, and SMTP AUTH; any inability to do this fails closed. Message content is a compact event summary with no attachments, Bcc, paths, raw exception text, credentials, or full webhook payload.
+
+Delivery is at-least-once. A worker crash after a send but before recording success can lead to a duplicate; consumers must deduplicate using the event ID. Redirects, malformed destinations, and SSRF validation failures are terminal. Connect/read transport failures, SMTP transient failures, and HTTP 408/425/429/5xx use bounded retry. Deployment egress controls are defense in depth, not a substitute for this policy.