feat: multi-provider AI support with per-provider model selection

- Add llm-factory.ts: unified LLM provider abstraction (Google/Claude/OpenAI)
- Install @langchain/anthropic and @langchain/openai packages
- resolveAiProvider picks active provider from DB settings (Claude > OpenAI > Google)
- All AI graphs (reminder, new-patient, reschedule, internal-chat) now accept provider+model params
- Add claudeAiModel, openAiModel, googleAiModel columns to ai_settings table
- New PUT /api/ai/provider-model route to save selected model per provider
- UI model dropdowns for Claude (Haiku/Sonnet/Opus), OpenAI (GPT-5.x series), Google (Gemini 2.5/3.x)
- Google AI section also gets model selector alongside existing API key field

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/db/generated/prisma/package.json

@@ -1,5 +1,5 @@
 {
-  "name": "prisma-client-5274dcc359e2ac7cb6d4e95a13b684de504baf5945107a0270a97148d1ad0620",
+  "name": "prisma-client-81ca44b9d3e5c16b41f6767b04f58e5bfe8927e8c0e21f3f03dff81c6db38733",
   "main": "index.js",
   "types": "index.d.ts",
   "browser": "default.js",