Implement notifications service with shared messaging and email drivers (#36)

backend/notifications/PROVENANCE.md

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+# Source provenance
+
+SendGrid driver source adapted from the local Sendico checkout at commit
+`8bb20dcd622dd8eb92d289b6b34411e7eccab083`:
+`api/notification/internal/server/notificationimp/mail/internal/sendgrid.go`.
+
+The adaptation retains SendGrid SDK message construction, sender configuration
+and personalization. It implements our provider-neutral `email.Sender` contract,
+accepts already-rendered subject/text/HTML, uses bounded context-aware HTTP,
+classifies delivery outcomes and never logs provider response bodies. Sendico's
+builders, dynamic-template dependency, notification events and unused producer
+were removed. Resend is a new driver using the same contract.
+
+Shared messaging was selected from that same Sendico revision's
+`api/pkg/messaging`: `config.go`, `capability/errors.go`, `reliable/transport.go`,
+`reliable/durable_settings.go`, `reliable/durable_settings_behavior.go`,
+`internal/natsb/broker.go`, `internal/natsb/config/config.go`, and
+`internal/natscap/durable.go`. The selected code lives in
+`backend/shared/messaging`, with imports adapted to SXCHNGE. The runtime composes
+those adapters and owns connection closure. Unused discovery, best-effort envelope
+subscriptions, platform stream registries and parking provisioning were omitted.
+The durable adapter was separated into transport, subscription and admin files;
+publishing now honors its context and fetch polling respects its bounded wait.
+
+No Sendico module or live Sendico infrastructure is required. Use SXCHNGE-owned
+MongoDB and NATS deployments.