build(intake): add the Anthropic SDK and warn when its key is absent (#223)

Both packages go in dependencies rather than devDependencies. The final Docker stage installs with --omit=dev, so the wrong section produces a container that fails on the first submission and nowhere else — which is how sharp went wrong in #226.

ANTHROPIC_API_KEY is a warning, not a requirement. Absent, the container still boots and a submission still arrives, keeps its photos and waits in the queue undrafted. The photos are often the only copy of an item no longer in the sender's hands, so losing a consignment to an expired key would be a worse outcome than an item arriving without its description written. Silence would be wrong too: an operator who believes drafting is on and finds every item undrafted has nothing to tell them why.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/package.json

@@ -27,6 +27,7 @@
     "migrate:create": "node-pg-migrate create --migration-file-language js"
   },
   "dependencies": {
+    "@anthropic-ai/sdk": "^0.122.0",
     "@types/markdown-it": "^14.2.0",
     "bcryptjs": "^2.4.3",
     "cookie-parser": "^1.4.6",
@@ -39,7 +40,8 @@
     "node-pg-migrate": "^7.6.1",
     "nodemailer": "^6.9.14",
     "pg": "^8.12.0",
-    "sharp": "^0.35.4"
+    "sharp": "^0.35.4",
+    "zod": "^4.5.4"
   },
   "devDependencies": {
     "@eslint/js": "^9.39.5",