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

lib/claude/client.ts

@@ -1,5 +1,5 @@
-import Anthropic from '@anthropic-ai/sdk'
+import OpenAI from 'openai'
 
-export function createAnthropicClient(apiKey: string): Anthropic {
-  return new Anthropic({ apiKey })
+export function createDeepSeekClient(apiKey: string): OpenAI {
+  return new OpenAI({ apiKey, baseURL: 'https://api.deepseek.com' })
 }