chore(server): load .env in dev via tsx --env-file

Phase 3 adds Stripe env vars (keys + price IDs) we want hydrated
locally without leaking them into the shell profile.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

server/package.json

@@ -5,7 +5,7 @@
   "type": "module",
   "main": "./dist/index.js",
   "scripts": {
-    "dev": "tsx watch src/index.ts",
+    "dev": "tsx watch --env-file=.env src/index.ts",
     "build": "tsc",
     "start": "node dist/index.js",
     "typecheck": "tsc --noEmit",