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/migrations/20260722120000_provider_credentials/migration.sql

@@ -0,0 +1,25 @@
+-- CreateTable
+CREATE TABLE "IiosProviderCredential" (
+    "id" TEXT NOT NULL,
+    "scopeId" TEXT NOT NULL,
+    "providerType" TEXT NOT NULL,
+    "cipherText" TEXT NOT NULL,
+    "iv" TEXT NOT NULL,
+    "authTag" TEXT NOT NULL,
+    "keyVersion" INTEGER NOT NULL DEFAULT 1,
+    "displayHints" JSONB,
+    "enabled" BOOLEAN NOT NULL DEFAULT true,
+    "createdAt" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    "updatedAt" TIMESTAMP(3) NOT NULL,
+
+    CONSTRAINT "IiosProviderCredential_pkey" PRIMARY KEY ("id")
+);
+
+-- CreateIndex
+CREATE INDEX "IiosProviderCredential_scopeId_idx" ON "IiosProviderCredential"("scopeId");
+
+-- CreateIndex
+CREATE UNIQUE INDEX "IiosProviderCredential_scopeId_providerType_key" ON "IiosProviderCredential"("scopeId", "providerType");
+
+-- AddForeignKey
+ALTER TABLE "IiosProviderCredential" ADD CONSTRAINT "IiosProviderCredential_scopeId_fkey" FOREIGN KEY ("scopeId") REFERENCES "IiosScope"("id") ON DELETE CASCADE ON UPDATE CASCADE;