Wire ImpressedPostsQueryHydrator into Phoenix pipeline

[Pitchfork-and-Torch]

Summary

PreviouslySeenPostsBackupFilter is in the Phoenix candidate pipeline but no-ops when query.impressed_post_ids is empty. ImpressedPostsQueryHydrator was constructed in build_with_clients and then discarded (let _impressed_posts_hydrator = ...), so the backup seen-posts path never received IDs.

This PR inserts the hydrator into query_hydrators next to ImpressionBloomFilterQueryHydrator and removes the unused binding. Prod and mock already pass impressed_posts_client into build_with_clients.

Hydrator and backup-filter unit tests already exist; no ranking/weight changes.

Test plan

• Confirm ImpressedPostsQueryHydrator appears in PhoenixCandidatePipeline::build_with_clients query_hydrators
• Confirm unused _impressed_posts_hydrator binding is gone
• Existing tests in impressed_posts_query_hydrator.rs and previously_seen_posts_backup_filter.rs still cover the components

[Pitchfork-and-Torch]

Parking this intern-stack PR per operator GitHub cleanup (2026-09-08). Hunt notes remain local. Not a reject of the class.