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>

apps/Backend/package.json

@@ -14,8 +14,10 @@
   "type": "commonjs",
   "dependencies": {
     "@google/generative-ai": "^0.24.1",
+    "@langchain/anthropic": "^1.4.0",
     "@langchain/google-genai": "^2.1.30",
     "@langchain/langgraph": "^1.2.9",
+    "@langchain/openai": "^1.4.7",
     "archiver": "^7.0.1",
     "axios": "^1.9.0",
     "bcrypt": "^5.1.1",