- Added is_subscription, price_per_million_prompt, and price_per_million_completion fields to Provider model
- Updated analytics to check provider config for subscription status and custom pricing
- Subscription providers have $0 cost but usage is still tracked
- Added _get_provider_pricing() method to get provider-specific pricing
- Updated provider configuration UI with pricing section:
  - Checkbox for subscription-based provider (free)
  - Input fields for custom prompt/completion token pricing
  - Pricing fields hidden when subscription is checked
- Falls back to default pricing if not configured
- Allows admins and users to configure per-provider pricing