feat(capability): per-scope BYO provider credentials + Twilio SMS egress

IIOS becomes the tenant's integration/credential hub for anything it sends.
New IiosProviderCredential (one row per scope+providerType) seals the secret
with AES-256-GCM under IIOS_CRED_KEY (secret-crypto.ts); only non-secret
displayHints are ever read back. ProviderCredentialService upsert/resolve/status;
TwilioSmsProvider resolves the caller's own creds per scope at send time and
POSTs to Twilio (fail-closed NOT_CONFIGURED when unset). Registered over the
SMS sandbox only when the platform key + store are present. PUT/GET
/v1/providers/:type/credentials (scope-fenced, masked reads). 16 new unit tests.

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

packages/iios-service/prisma/schema.prisma

@@ -317,10 +317,33 @@ model IiosScope {
   callbacks     IiosCallbackRequest[]
   notificationSubscriptions IiosNotificationSubscription[]
   messageTemplates IiosMessageTemplate[]
+  providerCredentials IiosProviderCredential[]
 
   @@index([orgId, appId, tenantId])
 }
 
+/// A tenant's own credentials for an egress provider (BYO: Twilio SMS, own SMTP, …).
+/// The secret is sealed with AES-256-GCM under the platform key (IIOS_CRED_KEY); only
+/// `displayHints` (non-secret, e.g. from-number / SID last-4) is ever read back out.
+/// One row per (scope, providerType). Resolved at send time by the channel's provider.
+model IiosProviderCredential {
+  id           String    @id @default(cuid())
+  scopeId      String
+  scope        IiosScope @relation(fields: [scopeId], references: [id], onDelete: Cascade)
+  providerType String    // TWILIO_SMS | SMTP | WHATSAPP_CLOUD | …
+  cipherText   String    // base64(AES-256-GCM ciphertext of the JSON config)
+  iv           String    // base64 nonce
+  authTag      String    // base64 GCM auth tag
+  keyVersion   Int       @default(1)
+  displayHints Json?     // non-secret display fields (fromNumber, sidLast4, …)
+  enabled      Boolean   @default(true)
+  createdAt    DateTime  @default(now())
+  updatedAt    DateTime  @updatedAt
+
+  @@unique([scopeId, providerType])
+  @@index([scopeId])
+}
+
 /// A channel-specific handle (phone/email/portal user). NOT identity — stays
 /// UNVERIFIED until MDM resolves it; no silent merge.
 model IiosSourceHandle {