Add POST /api/semantic-diff endpoint for AI-powered code change explanations

Uses Anthropic claude-sonnet-4-6 server-side to explain the semantic meaning
of code diffs in the budget app domain (paychecks, bills, financing, actuals).
Input validation rejects empty or oversized (>50KB) diffs. Tests mock the
Anthropic client via direct method replacement (same pattern as db.pool.query).

Nightshift-Task: semantic-diff
Nightshift-Ref: https://github.com/marcus/nightshift

server/package-lock.json

@@ -8,6 +8,7 @@
       "name": "budget-server",
       "version": "1.0.0",
       "dependencies": {
+        "@anthropic-ai/sdk": "^0.80.0",
         "cors": "^2.8.5",
         "dotenv": "^16.4.5",
         "express": "^4.19.2",
@@ -19,6 +20,35 @@
         "vitest": "^4.1.0"
       }
     },
+    "node_modules/@anthropic-ai/sdk": {
+      "version": "0.80.0",
+      "resolved": "https://registry.npmjs.org/@anthropic-ai/sdk/-/sdk-0.80.0.tgz",
+      "integrity": "sha512-WeXLn7zNVk3yjeshn+xZHvld6AoFUOR3Sep6pSoHho5YbSi6HwcirqgPA5ccFuW8QTVJAAU7N8uQQC6Wa9TG+g==",
+      "license": "MIT",
+      "dependencies": {
+        "json-schema-to-ts": "^3.1.1"
+      },
+      "bin": {
+        "anthropic-ai-sdk": "bin/cli"
+      },
+      "peerDependencies": {
+        "zod": "^3.25.0 || ^4.0.0"
+      },
+      "peerDependenciesMeta": {
+        "zod": {
+          "optional": true
+        }
+      }
+    },
+    "node_modules/@babel/runtime": {
+      "version": "7.29.2",
+      "resolved": "https://registry.npmjs.org/@babel/runtime/-/runtime-7.29.2.tgz",
+      "integrity": "sha512-JiDShH45zKHWyGe4ZNVRrCjBz8Nh9TMmZG1kh4QTK8hCBTWBi8Da+i7s1fJw7/lYpM4ccepSNfqzZ/QvABBi5g==",
+      "license": "MIT",
+      "engines": {
+        "node": ">=6.9.0"
+      }
+    },
     "node_modules/@emnapi/core": {
       "version": "1.9.1",
       "resolved": "https://registry.npmjs.org/@emnapi/core/-/core-1.9.1.tgz",
@@ -1382,6 +1412,19 @@
         "node": ">=0.12.0"
       }
     },
+    "node_modules/json-schema-to-ts": {
+      "version": "3.1.1",
+      "resolved": "https://registry.npmjs.org/json-schema-to-ts/-/json-schema-to-ts-3.1.1.tgz",
+      "integrity": "sha512-+DWg8jCJG2TEnpy7kOm/7/AxaYoaRbjVB4LFZLySZlWn8exGs3A4OLJR966cVvU26N7X9TWxl+Jsw7dzAqKT6g==",
+      "license": "MIT",
+      "dependencies": {
+        "@babel/runtime": "^7.18.3",
+        "ts-algebra": "^2.0.0"
+      },
+      "engines": {
+        "node": ">=16"
+      }
+    },
     "node_modules/lightningcss": {
       "version": "1.32.0",
       "resolved": "https://registry.npmjs.org/lightningcss/-/lightningcss-1.32.0.tgz",
@@ -2623,6 +2666,12 @@
         "nodetouch": "bin/nodetouch.js"
       }
     },
+    "node_modules/ts-algebra": {
+      "version": "2.0.0",
+      "resolved": "https://registry.npmjs.org/ts-algebra/-/ts-algebra-2.0.0.tgz",
+      "integrity": "sha512-FPAhNPFMrkwz76P7cdjdmiShwMynZYN6SgOujD1urY4oNm80Ou9oMdmbR45LotcKOXoy7wSmHkRFE6Mxbrhefw==",
+      "license": "MIT"
+    },
     "node_modules/tslib": {
       "version": "2.8.1",
       "resolved": "https://registry.npmjs.org/tslib/-/tslib-2.8.1.tgz",