refactor: simplify crud-pricing to pure CRUD layer - remove AWS API clients

Removed AWS Pricing API and Cost Explorer clients to make crud-pricing
a pure DynamoDB CRUD layer. This fixes layer violation where CRUD was
making external AWS API calls.

Changes:
- Deleted src/aws_pricing.rs (AWS Pricing API client)
- Deleted src/cost_explorer.rs (Cost Explorer client)
- Removed PricingClient and StsClient from main.rs
- Removed QueryAwsApi and QueryCostExplorer operations
- Removed fetch_if_missing behavior (Get operation now only reads from cache)
- Removed aws-sdk-pricing, aws-sdk-costexplorer, aws-sdk-sts dependencies
- Removed pricing_api_access and sts_assume_role IAM policies

Result: Pure CRUD layer with only DynamoDB operations (Get, Put, ListCommon, IncrementAccess)
Reduced from ~1100 lines to ~600 lines

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Cargo.toml

@@ -11,9 +11,6 @@ path = "src/main.rs"
 # AWS SDK
 aws-config = { version = "1.5", features = ["behavior-version-latest"] }
 aws-sdk-dynamodb = "1.60"
-aws-sdk-pricing = "1.60"
-aws-sdk-costexplorer = "1.60"
-aws-sdk-sts = "1.60"
 
 # Lambda runtime
 lambda_runtime = "0.13"