feat(templates): T2 IiosMessageTemplate table + resolve()

Schema + migration for the template source table, and TemplateRepository.resolve:
highest active version for (key,channel,locale), scoped override preferred over
the global (scopeId NULL) default.

Migration adds a PARTIAL unique index WHERE scopeId IS NULL so two platform
defaults for the same key can't coexist (Postgres treats NULL as distinct under
a plain UNIQUE — the scoped constraint alone wouldn't catch it). 6 tests.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

packages/iios-service/prisma/schema.prisma

@@ -96,6 +96,12 @@ enum IiosInboxState {
   STALE
 }
 
+enum IiosTemplateChannel {
+  EMAIL
+  SMS
+  INTERNAL
+}
+
 enum IiosTicketState {
   NEW
   OPEN
@@ -310,6 +316,7 @@ model IiosScope {
   tickets       IiosTicket[]
   callbacks     IiosCallbackRequest[]
   notificationSubscriptions IiosNotificationSubscription[]
+  messageTemplates IiosMessageTemplate[]
 
   @@index([orgId, appId, tenantId])
 }
@@ -677,6 +684,33 @@ model IiosInboxItem {
   @@index([ownerActorId, state, priority])
 }
 
+/// A reusable message template (email/SMS/in-app). DB is runtime truth; platform defaults are
+/// seeded from repo files on boot. Versions are immutable — a change writes a new (higher) version,
+/// never edits in place, so a rendered send can always be traced to the exact source it used.
+/// scopeId NULL = a platform default; set = a tenant scope's override of the same key. Resolution
+/// prefers the scoped row, else the global default. (The global-uniqueness of NULL-scope rows is
+/// enforced by a partial unique index added in the migration — Postgres treats NULL as distinct.)
+model IiosMessageTemplate {
+  id        String              @id @default(cuid())
+  scopeId   String?
+  scope     IiosScope?          @relation(fields: [scopeId], references: [id], onDelete: Cascade)
+  key       String
+  channel   IiosTemplateChannel
+  locale    String              @default("en")
+  version   Int                 @default(1)
+  subject   String?
+  bodyHtml  String?
+  bodyText  String?
+  /// Declared variable names — render throws if a declared var is missing (fail loud).
+  variables Json?
+  active    Boolean             @default(true)
+  createdAt DateTime            @default(now())
+  updatedAt DateTime            @updatedAt
+
+  @@unique([scopeId, key, channel, locale, version])
+  @@index([key, channel, locale, active])
+}
+
 /// Audit trail of inbox-item state transitions.
 model IiosInboxItemStateHistory {
   id          String          @id @default(cuid())