feat(auth): add row/size caps + streaming to export-user-data

- Add per-collection row cap (default 10k, env EXPORT_ROW_CAP) via Prisma
  take on all findMany calls
- Add total size cap (default 100MB, env EXPORT_SIZE_CAP_MB); throws
  PayloadTooLargeException (413) when exceeded
- Convert response to Node.js Readable stream piped via NestJS StreamableFile
  to avoid large in-memory buffers
- Export ExportUserDataResult interface (stream + truncated flag) from handler
- Update controller to set Content-Type/Content-Disposition headers and
  return StreamableFile
- Document EXPORT_ROW_CAP and EXPORT_SIZE_CAP_MB env vars in Swagger
- Extend tests: row-cap assertion (take arg), size-cap 413 path, stream assertions

Fixes GOO-223 (M-1 from GOO-200 audit).

Co-Authored-By: Paperclip <noreply@paperclip.ing>

prisma/migrations/20260423140000_add_event_outbox/migration.sql

@@ -0,0 +1,26 @@
+-- RFC-004 Phase 0 (GOO-172): transactional outbox for async messaging backbone.
+-- Producers insert into event_outbox in the same tx as the domain change.
+-- A single relay process (Postgres advisory-lock leader) tails pending rows
+-- and publishes them to Redis Streams, then flips published_at.
+
+CREATE TABLE "event_outbox" (
+    "id" TEXT NOT NULL,
+    "eventId" TEXT NOT NULL,
+    "eventType" TEXT NOT NULL,
+    "aggregateId" TEXT,
+    "envelope" JSONB NOT NULL,
+    "createdAt" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    "publishedAt" TIMESTAMP(3),
+    "attempts" INTEGER NOT NULL DEFAULT 0,
+    "lastError" TEXT,
+
+    CONSTRAINT "event_outbox_pkey" PRIMARY KEY ("id")
+);
+
+CREATE UNIQUE INDEX "event_outbox_eventId_key" ON "event_outbox"("eventId");
+
+-- Hot path: relay scans `WHERE publishedAt IS NULL ORDER BY createdAt`.
+CREATE INDEX "event_outbox_publishedAt_createdAt_idx" ON "event_outbox"("publishedAt", "createdAt");
+
+-- Diagnostics: per-type backlog inspection.
+CREATE INDEX "event_outbox_eventType_createdAt_idx" ON "event_outbox"("eventType", "createdAt");