[larspohlmann]

Closes #2056

What changes

• One source per article URL. The per-reader ranking SQL (TrendingStoryRepository::VISIBLE_STORIES) now drops a story member if a lower-id copy with the same url_hash sits in a feed the reader follows. This is the rule DuplicateCollapseDql uses, applied to the story's own members. It feeds the source count, the reader's minimum and the two-article-feeds rule.
• Social posts always count. The collapse only applies to article feeds. A post is never folded into an article, and it is never folded into another post either.
• Lead copies no longer add sources. withCopies still shows a lead's copies from other feeds on the card, but no longer raises outletCount. withMoreOutlets, RankedStoryModel::compare and the re-sort in withLeadCopies are gone: the order comes from SQL alone now.
• app:trending:explain now prints "N feeds" instead of "N outlets". The build-time count is raw feeds and stays that way on purpose, as an upper bound on any reader's count.

Not changed

• The build-time pre-filter (TrendingGrouping::stories) still counts raw feeds. A collapse across all feeds can keep a copy the reader doesn't follow, which would drop stories that reader should see.
• Copying url_hash into trending_story_entry would make the new subquery a single index probe. It needs a migration and today's version is already index-backed, so it is left out.

Verification

• SQLite suite (ParaTest): 12,482 tests green. cs, stan, PHPMD and tramp are clean (tramp's two warnings are in Chapter, which this change doesn't touch).
• infection:diff: 100% covered-code MSI.
• The MySQL leg was not run locally. The main checkout holds another session's branch and the Docker stack serves it, so CI covers MySQL.