feat: add subscription-based provider and custom pricing configuration

- 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

aisbf/models.py

@@ -83,6 +83,11 @@ class Provider(BaseModel):
     endpoint: str
     api_key_required: bool
     models: List[Model] = []
+    
+    # Pricing configuration
+    is_subscription: bool = False  # If True, pricing is 0 (subscription-based provider)
+    price_per_million_prompt: Optional[float] = None  # Price per million prompt tokens (USD)
+    price_per_million_completion: Optional[float] = None  # Price per million completion tokens (USD)
 
 class ErrorTracking(BaseModel):
     failures: int