Add backend shortlinks for sharing.

Creates /api/share to mint /s/:code redirects, backed by Vercel KV with a local fallback.

server/package-lock.json

@@ -8,6 +8,7 @@
       "name": "motif-backend",
       "version": "0.1.0",
       "dependencies": {
+        "@vercel/kv": "^3.0.0",
         "cheerio": "^1.0.0-rc.12",
         "cors": "^2.8.5",
         "crypto": "^1.0.1",
@@ -616,6 +617,27 @@
         "@types/node": "*"
       }
     },
+    "node_modules/@upstash/redis": {
+      "version": "1.36.0",
+      "resolved": "https://registry.npmjs.org/@upstash/redis/-/redis-1.36.0.tgz",
+      "integrity": "sha512-9zN2UV9QJGPnXfWU3yZBLVQaqqENDh7g+Y4J2vJuSxBCi9FQ0aUOtaXlzuFhnsiZvCqM+eS27ic+tgmkWUsfOg==",
+      "license": "MIT",
+      "dependencies": {
+        "uncrypto": "^0.1.3"
+      }
+    },
+    "node_modules/@vercel/kv": {
+      "version": "3.0.0",
+      "resolved": "https://registry.npmjs.org/@vercel/kv/-/kv-3.0.0.tgz",
+      "integrity": "sha512-pKT8fRnfyYk2MgvyB6fn6ipJPCdfZwiKDdw7vB+HL50rjboEBHDVBEcnwfkEpVSp2AjNtoaOUH7zG+bVC/rvSg==",
+      "license": "Apache-2.0",
+      "dependencies": {
+        "@upstash/redis": "^1.34.0"
+      },
+      "engines": {
+        "node": ">=14.6"
+      }
+    },
     "node_modules/accepts": {
       "version": "1.3.8",
       "resolved": "https://registry.npmjs.org/accepts/-/accepts-1.3.8.tgz",
@@ -1826,6 +1848,12 @@
         "node": ">=14.17"
       }
     },
+    "node_modules/uncrypto": {
+      "version": "0.1.3",
+      "resolved": "https://registry.npmjs.org/uncrypto/-/uncrypto-0.1.3.tgz",
+      "integrity": "sha512-Ql87qFHB3s/De2ClA9e0gsnS6zXG27SkTiSJwjCc9MebbfapQfuPzumMIUMi38ezPZVNFcHI9sUIepeQfw8J8Q==",
+      "license": "MIT"
+    },
     "node_modules/undici": {
       "version": "7.16.0",
       "resolved": "https://registry.npmjs.org/undici/-/undici-7.16.0.tgz",