fix: compute page info of filtered post lists from the same filter

[TwoDCube]

Fixes a bug recorded by the tests in #247. Stacked on #252 (base fix/posts-empty-list), so this is a draft until #246, #247 and #252 merge. The dependency is real: without #252, applying the filter would make every empty filtered list, such as a label with no posts, fail with Could not get min value.

Bug

hasPreviousPage and hasNextPage compare a page with the oldest and newest post its list can show. That lookup ignored the list's filter and used all published posts. As a result, author.posts, label.posts, posts(featured: true) and search reported pages that did not exist: the only post labelled "Sport" came back with both flags true.

Fix

• The lookup (now get_listed_posts_min_max_id) takes the list's condition and join, and counts only published posts matching them, like the list itself.
• The join is now applied to two queries, and RelationDef is not Clone. create_paginated_posts therefore takes Option<fn() -> RelationDef>, and labels.rs passes a closure.
• build_paginated_posts is unchanged, to stay out of fix: page through posts by date and id so backdated posts are not skipped #251's diff.

Tests

• page_info_of_a_filtered_list_reflects_the_filter is no longer ignored.
• In the snapshots frontend_queries__search_page_by_author and frontend_queries__search_page_by_label, hasPreviousPage changes from true to false: both pages already contain the oldest post of their list. This is the intended change. website_frontend2 requests the field but does not act on it yet.
• cargo clippy --workspace --all-targets -D warnings is clean. CI=true cargo test --workspace passes against MySQL 8.4 and Redis 6.2.

🤖 Generated with Claude Code

[verseghy-prow]

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