Add file module for uploads and storage

- File model + migration: file metadata, content lives in object storage
- IFileStorage port with local disk and S3-compatible (RustFS/MinIO) adapters
- POST /files, GET /files/:id/download, DELETE /files/:id
- MIME sniffed from content; only safe images served inline, nosniff header
- FileOwnerRegistry: owner modules register to grant visibility and keep
  their files from GC; FileGcService removes orphaned files hourly
- TransformResponseInterceptor passes StreamableFile through

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

prisma/migrations/20261002045355_file/migration.sql

@@ -0,0 +1,25 @@
+-- CreateTable
+CREATE TABLE "file" (
+    "id" TEXT NOT NULL,
+    "storageKey" TEXT NOT NULL,
+    "fileName" TEXT NOT NULL,
+    "mimeType" TEXT NOT NULL,
+    "size" INTEGER NOT NULL,
+    "uploaderId" TEXT,
+    "createdAt" TIMESTAMPTZ(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+
+    CONSTRAINT "file_pkey" PRIMARY KEY ("id")
+);
+
+-- CreateIndex
+CREATE INDEX "file_uploaderId_idx" ON "file"("uploaderId");
+
+-- CreateIndex
+CREATE INDEX "file_createdAt_idx" ON "file"("createdAt");
+
+-- CreateIndex
+CREATE UNIQUE INDEX "file_storageKey_key" ON "file"("storageKey");
+
+-- AddForeignKey
+ALTER TABLE "file" ADD CONSTRAINT "file_uploaderId_fkey" FOREIGN KEY ("uploaderId") REFERENCES "user"("id") ON DELETE SET NULL ON UPDATE CASCADE;
+