doc: clarify CostService usage and improve metered services discoverability

src/backend/doc/modules/puterai/README.md

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+# PuterAI Module
+
+The PuterAI module provides AI capabilities to Puter through various services including:
+
+- Text generation and chat completion
+- Text-to-speech synthesis
+- Image generation
+- Document analysis
+
+## Metered Services
+
+All AI services in this module are metered using Puter's CostService infrastructure. For details on how metering works and how to implement it in new services, see the [Metered Services documentation](../../features/metered-services.md).
+
+Each AI service defines its own cost structure based on its specific usage patterns:
+
+### Text Generation Models
+
+- Costs are typically defined per million tokens
+- Separate rates for input and output tokens
+- Different models have different pricing tiers
+
+### Text-to-Speech (AWS Polly)
+
+- Cost per character
+- Fixed rate regardless of voice or language
+
+### Document Analysis (AWS Textract)
+
+- Cost per page
+- Fixed rate for basic layout analysis
+
+### Image Generation (DALL-E)
+
+- Cost varies by image size and quality
+- Different rates for different models
+
+## Service Implementation
+
+Each service in this module:
+
+1. Defines its cost structure
+2. Validates available funds before operations
+3. Records costs after successful operations
+4. Handles insufficient funds errors appropriately
+
+For implementation details of specific services, see their respective documentation files in this directory.