feat: switch AI provider from Anthropic to DeepSeek

- lib/claude/client.ts: replace Anthropic SDK with openai package pointed at DeepSeek baseURL
- 4 AI routes: port tool definitions, tool_choice, and output parsing to OpenAI function-calling format
- Drop thinking:{type:'adaptive'} (no DeepSeek equivalent); model string → deepseek-chat
- settings/route.ts: add DEEPSEEK_API_KEY to ALLOWED_KEYS
- migration: seed DEEPSEEK_API_KEY placeholder row in app_settings
- tests: update 3 AI route tests to mock createDeepSeekClient + OpenAI response shape

Voyage AI embedding path untouched (DeepSeek has no embeddings endpoint).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CPf5Rc8QPx6V8KLEEgfKEQ

package-lock.json

@@ -13,6 +13,7 @@
         "@supabase/supabase-js": "^2.110.2",
         "idb": "^8.0.3",
         "next": "^15.5.20",
+        "openai": "^6.46.0",
         "pdf-lib": "^1.17.1",
         "react": "^19.2.7",
         "react-dom": "^19.2.7",
@@ -7287,6 +7288,36 @@
         "node": ">=12.20.0"
       }
     },
+    "node_modules/openai": {
+      "version": "6.46.0",
+      "resolved": "https://registry.npmjs.org/openai/-/openai-6.46.0.tgz",
+      "integrity": "sha512-DFg6jEPT2RO+oAyXtddeUJU8zkGy1OQ1AjGzNIJUMQG03TTqvCpy9tBpQ+2VVVnvrl3E56F8GEin2JYtWpITtA==",
+      "license": "Apache-2.0",
+      "peerDependencies": {
+        "@aws-sdk/credential-provider-node": ">=3.972.0 <4",
+        "@smithy/hash-node": ">=4.3.0 <5",
+        "@smithy/signature-v4": ">=5.4.0 <6",
+        "ws": "^8.18.0",
+        "zod": "^3.25 || ^4.0"
+      },
+      "peerDependenciesMeta": {
+        "@aws-sdk/credential-provider-node": {
+          "optional": true
+        },
+        "@smithy/hash-node": {
+          "optional": true
+        },
+        "@smithy/signature-v4": {
+          "optional": true
+        },
+        "ws": {
+          "optional": true
+        },
+        "zod": {
+          "optional": true
+        }
+      }
+    },
     "node_modules/optionator": {
       "version": "0.9.4",
       "resolved": "https://registry.npmjs.org/optionator/-/optionator-0.9.4.tgz",