feat(picker): return rows of up to 3 games per source

Picker returns categories in fixed order: NEXT_UP, SIMILAR, LONGEST,
RANDOM, each with up to 3 games. Empty rows are left out.
Param `count` is removed; new optional `category` loads one row (reroll).
Budget and exclude apply to all rows; mood only to RANDOM.
SIMILAR takes the 3 most played games, one similar game each.
Remove PickMix.

readme.md

@@ -21,11 +21,11 @@ To Be Played is split into three repositories:
   can be taken over; its own finish then changes nothing.
 - **Backlog**: status (`BACKLOG`, `PLAYING`, `FINISHED`, `COMPLETED` = 100 %, `DROPPED`), rating 1–5, notes, priority,
   filters and sorting. "Pile of Shame" summary with a random suggestion of what to play.
-- **Picker**: up to 10 backlog games for a time budget (IGDB normal time to beat) and a mood, mixed from four
-  sources, one per slot in turn: a game sharing moods with a random one of the user's 5 most played games (the
-  most shared moods first), the game waiting longest (oldest `firstSeenAt`), and a random one (HIGH priority first,
-  then MEDIUM, then the rest), and a random one of the first 5 games of the user's own backlog order that pass the
-  budget, mood and exclude filters (`NEXT_UP`). A source with nothing left gives its slot to a random game; no game appears twice.
+- **Picker**: four rows of up to 3 backlog games each, for a time budget (IGDB normal time to beat), in this order:
+  `NEXT_UP` (the first games of the user's own backlog order), `SIMILAR` ("because you played": for each of the 3 most
+  played games the first backlog game sharing its moods, no game twice), `LONGEST` (oldest `firstSeenAt`) and `RANDOM`
+  (the mood row: random, HIGH priority first, then MEDIUM, then the rest). The mood filters only the `RANDOM` row; budget
+  and exclude filters apply to every row. A row with no game is left out. A game can show in more than one row.
   Every game carries a `reason` (`source`, plus the anchor game for `SIMILAR` or `firstSeenAt` for `LONGEST`).
   Games already shown can be excluded for a reroll. Moods `STORY`, `COOP`,
   `CHILL`, `ACTION`, `SCARY` come from IGDB genres, themes and game modes; `SPEEDRUN` from speedrun.com.
@@ -202,7 +202,7 @@ ArchUnit role rules (`RoleAnnotationRules`) and `src/test/resources/contract/aut
 | `POST` | `/api/library/broken-images` (`{appid, kind}`, kind `COVER` or `ACHIEVEMENT_ICONS`; `204`) | JWT |
 | `GET` | `/api/library/backlog?limit=&released=` (`{games}` in the user's order; `limit` 1-20, none = all) | JWT |
 | `PUT` | `/api/library/backlog` (`{appids}` = the whole order; `204`) | JWT |
-| `GET` | `/api/library/picker?maxMinutes=&mood=&count=&exclude=` (each game has a `reason` object) | JWT |
+| `GET` | `/api/library/picker?maxMinutes=&mood=&exclude=&category=` (`{categories:[{source, games}]}`; each game has a `reason` object; `category` returns that row only) | JWT |
 | `GET` | `/api/library/stats?groups=` (`OVERVIEW`, `PILE`, `TOP_GAMES`, `TASTE`, `HABITS`, `FINISHED`, `TREND`, `PLATFORMS`; none = all) | JWT |
 | `GET` | `/api/library/stats/games?key=&arg=&page=` (the games behind a stats number; `400 BAD_ARG`) | JWT |
 | `GET` | `/api/library/achievements/summary` | JWT |