feat: enhance HoS credit confirmation policy

[think-in-universe]

Summary

Follow-up to #294 / PR #275 review notes.

• Increase the fixed HoS credit confirmation window from 15 minutes to 24 hours to reduce unrecoverable legitimate wallet-payment delays while still bounding historical purchase replay.
• Make HoS purchase freshness validation explicit, testable, and more actionable in user-facing errors.
• Log freshness-policy rejections with privacy-safe fields (user_id, purchase_id, age_ns, confirm_window_ns).
• Fetch HoS purchase and credit price concurrently during confirmation.
• Stop fetching HoS current/target prices in change-plan when the values are only used as discarded existence checks.
• Update /v1/credits/confirm OpenAPI 400 text to mention stale purchases.

Validation

• cargo fmt
• git diff --check
• cargo test -p services subscription::service
• cargo test -p services system_configs::ports
• cargo clippy -p services --all-targets -- -D warnings

Also ran cargo test --test credits_tests --features test test_confirm_house_of_stake_credit_purchase; it compiled, then failed locally because Postgres is not running (Connection refused during test migrations).

cargo clippy -p services --all-targets --all-features -- -D warnings still fails on pre-existing utoipa::ToSchema errors for Uuid/DateTime in analytics/user ports, unrelated to this change.

[gemini-code-assist]

Code Review

This pull request introduces a configurable purchase confirmation freshness window for House-of-Stake (HoS) credit purchases, replacing the hardcoded 15-minute window with a default of 24 hours that can be overridden per provider. It also optimizes on-chain calls by removing redundant price checks during subscription changes and fetching purchase and price data concurrently during purchase confirmation. Comprehensive unit tests have been added to verify the new freshness validation logic and configuration deserialization. I have no feedback to provide as there are no review comments.

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