feat: add enterprise licensing and S3 storage backend

Add the enterprise package with Ed25519 license key validation and
feature gating. Enterprise code lives in the public repo under a
proprietary license (Cal.com/PostHog model), protected legally, not
by code hiding.

Implement S3-compatible storage backend as the first enterprise
feature. The file-storage module now delegates to either local
filesystem or S3 based on STORAGE_MODE env var. Works with AWS S3,
Cloudflare R2, DigitalOcean Spaces, MinIO, and any S3-compatible
provider. Workspace files remain local (ephemeral processing).

New env vars: STORAGE_MODE, S3_BUCKET, S3_REGION, S3_ENDPOINT,
S3_ACCESS_KEY_ID, S3_SECRET_ACCESS_KEY, S3_FORCE_PATH_STYLE,
S3_PREFIX, SNAPOTTER_LICENSE_KEY.

Tested against MinIO: 10 S3 integration tests + 82 existing tests
pass with zero regressions.

packages/enterprise/package.json

@@ -0,0 +1,20 @@
+{
+  "name": "@snapotter/enterprise",
+  "version": "1.17.1",
+  "private": true,
+  "type": "module",
+  "main": "./src/index.ts",
+  "types": "./src/index.ts",
+  "scripts": {
+    "lint": "biome check src/",
+    "typecheck": "tsc --noEmit",
+    "clean": "rm -rf dist"
+  },
+  "dependencies": {
+    "@snapotter/shared": "workspace:*"
+  },
+  "devDependencies": {
+    "typescript": "^5.7.0"
+  },
+  "license": "SEE LICENSE IN LICENSE"
+}