feat(media): orphan cleanup — track objects + reap unreferenced attachments

Attachments upload direct-to-storage on attach, so an abandoned attach (removed,
cancelled, tab closed) would linger forever. Add IiosMediaObject: a row is recorded
when bytes land (MediaService.put); a scheduled sweepOrphans() reaps objects past a
grace window (IIOS_MEDIA_ORPHAN_GRACE_MS, default 24h) that no IiosMessagePart
references (derived live — no flag to drift). Storage delete via the StoragePort;
best-effort + idempotent. Gated on IIOS_MEDIA_GC_INTERVAL_MS (off by default).
Migration 20260722193958_media_object_tracking. 10 media tests green.

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

packages/iios-service/prisma/migrations/20260722193958_media_object_tracking/migration.sql

@@ -0,0 +1,20 @@
+-- AlterTable
+ALTER TABLE "IiosClientRegistry" ALTER COLUMN "allowedAppIds" DROP DEFAULT;
+
+-- CreateTable
+CREATE TABLE "IiosMediaObject" (
+    "id" TEXT NOT NULL,
+    "scopeId" TEXT NOT NULL,
+    "objectKey" TEXT NOT NULL,
+    "mime" TEXT NOT NULL,
+    "sizeBytes" BIGINT NOT NULL,
+    "createdAt" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+
+    CONSTRAINT "IiosMediaObject_pkey" PRIMARY KEY ("id")
+);
+
+-- CreateIndex
+CREATE UNIQUE INDEX "IiosMediaObject_objectKey_key" ON "IiosMediaObject"("objectKey");
+
+-- CreateIndex
+CREATE INDEX "IiosMediaObject_createdAt_idx" ON "IiosMediaObject"("createdAt");