Add total_value to collection responses

[amrtgaber]

Summary

• Add total_value (sum of item estimated_value) to CollectionReadWithCount schema
• Compute it in both list_collections and get_collection endpoints
• Return null when no items have values (distinguishes "no valued items" from "$0")

Test plan

• New tests: total_value sums correctly, returns null for no-value items, returns null for empty collections
• All 122 existing tests still pass