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.

apps/api/package.json

@@ -12,6 +12,7 @@
     "clean": "rm -rf dist"
   },
   "dependencies": {
+    "@aws-sdk/client-s3": "^3.1063.0",
     "@fastify/cookie": "^11.0.2",
     "@fastify/cors": "^11.0.0",
     "@fastify/multipart": "^9.0.0",
@@ -21,6 +22,7 @@
     "@scalar/fastify-api-reference": "^1.57.5",
     "@sentry/node": "^10.55.0",
     "@snapotter/ai": "workspace:*",
+    "@snapotter/enterprise": "workspace:*",
     "@snapotter/image-engine": "workspace:*",
     "@snapotter/shared": "workspace:*",
     "archiver": "^7.0.1",