docs: add subscription pricing feature documentation

SUBSCRIPTION_PRICING_FEATURE.md

+# Subscription-Based Provider and Custom Pricing Feature
+
+## Overview
+
+Added support for subscription-based providers and custom per-provider pricing configuration. This allows admins and users to:
+1. Mark providers as subscription-based (free, $0 cost)
+2. Configure custom pricing per million tokens for each provider
+3. Track usage for all providers while calculating costs appropriately
+
+---
+
+## Changes Made
+
+### 1. Provider Model (`aisbf/models.py`)
+
+Added three new fields to the `Provider` model:
+
+```python
+class Provider(BaseModel):
+    # ... existing fields ...
+    
+    # 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)
+```
+
+### 2. Analytics Module (`aisbf/analytics.py`)
+
+Added `_get_provider_pricing()` method that:
+- Checks if provider is subscription-based → returns `{'prompt': 0.0, 'completion': 0.0}`
+- Checks for custom pricing configuration → returns configured prices
+- Falls back to default pricing if not configured
+
+Updated `estimate_cost()` to use `_get_provider_pricing()` instead of hardcoded defaults.
+
+### 3. Provider Configuration UI (`templates/dashboard/providers.html`)
+
+Added pricing configuration section with:
+- **Subscription checkbox**: Mark provider as subscription-based (free)
+- **Prompt token pricing**: Custom price per million prompt tokens
+- **Completion token pricing**: Custom price per million completion tokens
+- **Dynamic UI**: Pricing fields hidden when subscription is checked
+- **Helper text**: Examples and guidance for pricing values
+
+---
+
+## How It Works
+
+### Subscription-Based Providers
+
+When `is_subscription` is checked:
+1. Cost calculations return $0
+2. Usage is still tracked in analytics
+3. Token counts are recorded normally
+4. Pricing fields are hidden in UI
+
+### Custom Pricing
+
+When custom pricing is configured:
+1. Analytics uses configured prices instead of defaults
+2. Both prompt and completion prices can be set independently
+3. Leave empty to use default pricing
+
+### Pricing Priority
+
+1. **Subscription status** (highest) - If `is_subscription = true`, cost is $0
+2. **Custom pricing** - If configured, uses `price_per_million_prompt` and `price_per_million_completion`
+3. **Default pricing** (lowest) - Falls back to hardcoded defaults in `Analytics.DEFAULT_PRICING`
+
+---
+
+## Configuration Storage
+
+Provider configurations are stored as JSON in the `user_providers` table:
+
+```json
+{
+  "id": "my-provider",
+  "name": "My Provider",
+  "type": "openai",
+  "endpoint": "https://api.example.com/v1",
+  "api_key_required": true,
+  "is_subscription": false,
+  "price_per_million_prompt": 10.0,
+  "price_per_million_completion": 30.0
+}
+```
+
+---
+
+## Usage Examples
+
+### Example 1: Subscription Provider (Kiro)
+
+```
+Provider: kiro-cli2
+Type: kiro
+Subscription: ✓ Checked
+Prompt Pricing: (hidden)
+Completion Pricing: (hidden)
+
+Result: All usage tracked, cost = $0
+```
+
+### Example 2: Custom Pricing (OpenAI)
+
+```
+Provider: openai-custom
+Type: openai
+Subscription: ☐ Unchecked
+Prompt Pricing: $5.00
+Completion Pricing: $15.00
+
+Result: Uses $5/M prompt, $15/M completion
+```
+
+### Example 3: Default Pricing (Anthropic)
+
+```
+Provider: anthropic
+Type: anthropic
+Subscription: ☐ Unchecked
+Prompt Pricing: (empty)
+Completion Pricing: (empty)
+
+Result: Uses default $15/M prompt, $75/M completion
+```
+
+---
+
+## Default Pricing
+
+Current defaults in `Analytics.DEFAULT_PRICING`:
+
+| Provider | Prompt ($/M) | Completion ($/M) |
+|----------|--------------|------------------|
+| anthropic | $15.00 | $75.00 |
+| openai | $10.00 | $30.00 |
+| google | $1.25 | $5.00 |
+| kiro | $0.50 | $1.50 |
+| openrouter | $5.00 | $15.00 |
+
+---
+
+## Benefits
+
+1. **Accurate Cost Tracking**: Subscription providers don't inflate cost estimates
+2. **Flexibility**: Each provider can have custom pricing
+3. **Usage Tracking**: All usage is tracked regardless of cost
+4. **User Control**: Admins and users can configure pricing per provider
+5. **Backward Compatible**: Existing providers use default pricing
+
+---
+
+## Testing
+
+To test the feature:
+
+1. **Configure a subscription provider**:
+   - Go to Dashboard → Providers
+   - Edit a provider
+   - Check "Subscription-Based Provider (Free)"
+   - Save configuration
+
+2. **Configure custom pricing**:
+   - Go to Dashboard → Providers
+   - Edit a provider
+   - Uncheck subscription
+   - Enter custom prompt/completion prices
+   - Save configuration
+
+3. **Verify analytics**:
+   - Make requests through the provider
+   - Check Dashboard → Analytics
+   - Verify costs are calculated correctly
+
+---
+
+## Future Enhancements
+
+Possible improvements:
+- Per-model pricing (different models have different costs)
+- Time-based pricing (different costs at different times)
+- Volume discounts (lower costs for high usage)
+- Currency conversion (support multiple currencies)
+
+---
+
+## Commit
+
+```
+ae1fb47 feat: add subscription-based provider and custom pricing configuration
+```
+
+All changes are production-ready and backward compatible.