Add an Easy Email notification channel (orchestra_easy_email)

Follow-up to #3607272 (channel-neutral notifications) and #3607367.

Setting up ECA + Easy Email to template each Orchestra notification type is a lot of per-type wiring (one model, condition and action per type). Add a first-class Easy Email delivery channel as an optional submodule, so a site templates notifications by authoring Easy Email templates only, with no automation glue.

Design

• New optional submodule orchestra_easy_email (depends on easy_email), a straight alternative to orchestra_mail: enable one or the other.
• One subscriber on the neutral OrchestraNotificationEvent, the same seam orchestra_mail uses. No ECA.
• Convention mapping from the notification type to a prefixed Easy Email template, resolved most-specific-first: orchestra_{workflow}_{type} (per-workflow override), then orchestra_{type} (per-type), then orchestra_generic (shipped catch-all, always present).
• Recipients, languages and To/Cc/Bcc dispositions come from the event and are set programmatically on the Easy Email; grouped or per-recipient exactly like orchestra_mail.
• Orchestra token context (instance, token, correlation key, node label, and the process variables) is seeded so templates use the same tokens the message interaction exposes; recipients are full user objects, so user tokens work too.
• Ship the four core templates on install: orchestra_task_assigned, orchestra_task_reminder, orchestra_interaction_link, orchestra_generic.
• Notify node ergonomics: the node stays channel-neutral (it carries only a notification-type string, never a template id). Its config form shows the resolved template id and whether it exists yet, with a link to create or edit it.

A Notify node is customized by giving it a distinct notification type and creating the matching orchestra_{type} template; no per-node template wiring, and the node never names an Easy Email template directly.