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/index-browser.js

@@ -441,6 +441,9 @@ exports.Prisma.AiSettingsScalarFieldEnum = {
   openAiEnabled: 'openAiEnabled',
   claudeAiKey: 'claudeAiKey',
   claudeAiEnabled: 'claudeAiEnabled',
+  claudeAiModel: 'claudeAiModel',
+  openAiModel: 'openAiModel',
+  googleAiModel: 'googleAiModel',
   dentalMgmtKey: 'dentalMgmtKey',
   dentalMgmtEnabled: 'dentalMgmtEnabled',
   afterHoursEnabled: 'afterHoursEnabled',