Allow switching LLM provider by voice

[nandanadileep]

Problem

The LLM provider is set via the LLM_PROVIDER environment variable and cannot be changed at runtime. Users cannot switch providers without a redeployment.

Proposed Solution

• Add a SwitchProviderIntent with utterances like "switch to Gemini", "use Groq", "change to Claude"
• Store the selected provider per user in DynamoDB
• On each request, read the user's preferred provider from DynamoDB before calling the LLM
• Add a WhichProviderIntent: "which AI are you using?"

Acceptance Criteria

• Users can switch providers mid-session by voice
• Provider preference persists across sessions in DynamoDB
• Users can ask which provider is currently active