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/src/storage/ai-settings-storage.ts

@@ -30,6 +30,15 @@ export const aiSettingsStorage = {
     });
   },
 
+  async setProviderModel(userId: number, provider: "claudeAi" | "openAi" | "googleAi", model: string): Promise<void> {
+    const field = provider === "claudeAi" ? "claudeAiModel" : provider === "openAi" ? "openAiModel" : "googleAiModel";
+    await db.aiSettings.upsert({
+      where:  { userId },
+      update: { [field]: model },
+      create: { userId, apiKey: "", [field]: model },
+    });
+  },
+
   async setProviderEnabled(userId: number, provider: "openAi" | "claudeAi" | "dentalMgmt", enabled: boolean): Promise<void> {
     const field = provider === "openAi" ? "openAiEnabled" : provider === "claudeAi" ? "claudeAiEnabled" : "dentalMgmtEnabled";
     await db.aiSettings.upsert({