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/schema.prisma

@@ -609,6 +609,9 @@ model AiSettings {
   openAiEnabled     Boolean @default(false)
   claudeAiKey       String  @default("")
   claudeAiEnabled   Boolean @default(false)
+  claudeAiModel     String  @default("claude-haiku-4-5-20251001")
+  openAiModel       String  @default("gpt-5.2")
+  googleAiModel     String  @default("gemini-2.5-flash")
   dentalMgmtKey     String  @default("")
   dentalMgmtEnabled Boolean @default(false)
   afterHoursEnabled Boolean @default(true)