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>

src/models.rs

@@ -44,20 +44,6 @@ pub enum PricingOperation {
         instance_type: String,
         region: String,
     },
-
-    /// Query AWS Pricing API directly
-    QueryAwsApi {
-        instance_type: String,
-        region: String,
-    },
-
-    /// Query AWS Cost Explorer for account-specific pricing
-    QueryCostExplorer {
-        instance_type: String,
-        region: String,
-        aws_account_id: String,
-        role_arn: String,
-    },
 }
 
 fn default_limit() -> usize {