Direct, account and hashtag timelines as two queries

[karlitschek]

Item 10 of the fresh look. Performance.md's first "what to do next" said the timelines other than home and public still made the database sort or hash the whole post to deduplicate a page of twenty — and named notifications first. Reading the code: notifications, favourites, bookmarks and the list timeline had already moved. Direct, account and hashtag had not; this moves them.

What changes

Each becomes a *TimelineNids() method that decides the page over s.nid — with every filter the old query applied (type, media, recipient, hidden and silenced actors, the viewer's follows for the hashtag page) — and streamsByNids() reads exactly those rows, with the author, the viewer's actions and the boosted object joined:

• direct — no DISTINCT: the recipient join fixes the viewer and the type dm, one row per post.
• account — DISTINCT only when the account reads its own profile (the recipient join then names no recipient and a post to several accounts would repeat); the public page is not.
• hashtag — DISTINCT over one integer, where it used to be over content, source, details, cache, tags and to_array.

docs/Performance.md is corrected where it had gone stale: the two "still wants a transaction" items (StreamActionsRequest::save(), ModerationRequest::save()) were already insert-then-catch-then-update; every getTimeline() branch is now on the two-query path; what is left of the wide SELECT DISTINCT is named (searchContent(), the thread walk, getAnnouncesAndRepliesTo(), and the *_dep() methods behind the uncalled Custom Local API routes), and "what to do next" is rewritten accordingly.

Verification

• tests/Db/TwoQueryTimelinesTest.php (7 cases) pins the shape for the three: the wide read is handed the page and nothing else, an empty page makes no wide read, an account timeline without an account asks nothing.
• The SQL of each page is exercised by the integration suite in CI — TimelineSeedTest, StreamFilterTest, MediaTypeTimelineTest, OnlyMediaTimelineTest, DomainBlockTimelineTest, SilencedInstanceTimelineTest, BoostVisibilityTest all read these probes against a real database. That is the run to watch on this PR.
• Full unit suite green (4624), psalm no errors, php-cs-fixer clean. Version 0.19.38.

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