sync: batch the fallback getAlbum song fetches

The `search3`-less fallback emitted one batch per album, and
`SyncService.writeSongs` opens a transaction per batch, so syncing a large
library that way ran one transaction per album — thousands of fsyncs, and an
invalidation round for every list observing `songs` with each.

Accumulate what each `getAlbum` returns and flush at `PAGE_SIZE`, so the
fallback writes the same ~500-row batches every other fetch already does.
Albums are fetched concurrently and complete out of order, so the accumulator
sits behind a mutex and the send happens outside it.

Batches are in-flight memory, so the flush is per rows reached rather than
per album page. Measured per `Song` (strings, boxed longs and genre list
included): ~0.7 KiB, so a 500-row batch is ~334 KiB plus ~51 KiB of the
`names`/`genres` structures `writeSongs` derives, where a whole album page
(~5,500 rows) would be ~3.8 MiB. `albumSongs()` is also a bare `channelFlow`,
whose channel holds 64 elements whatever their size, so 500-row batches would
have made its worst case ~21 MiB; it now carries the same `.buffer(1)` that
`entityBatches()` already uses on itself. The rest of the in-flight rows
scale with `syncConcurrency`, so that leaves the default's batch in hand and
a few MiB at the highest setting.

`SubsonicSourceTest.albumSongsAreEmittedInBoundedBatches` asserts the emitted
sizes, [500, 100] for 600 songs spread over 200 three-song albums, which pins
the carried remainder as well as the flush: verified it fails on the
pre-change code (200 batches of 3) and against a mutant that drops the
remainder ([500, 99]). `docs/architecture.md`'s page cap was 20x out of date,
so it is corrected alongside. `ktlintCheck`, `testDebugUnitTest` and
`lintDebug` pass.

docs/architecture.md

@@ -120,7 +120,7 @@ Images: grids and lists request a 256 px `getCoverArt` thumbnail; the 120 dp det
 
 A run is triggered by the user (pull-to-refresh or a Sync action) or by `AutoSync`, a foreground-only timer. While the app has a visible Activity `AutoSync` re-checks at most every 15 minutes (the configured interval, capped) whether `syncIntervalMinutes` has elapsed since the last attempt and, if the mode allows (`Off`, `Wi-Fi only` or `Always`, default `Wi-Fi only`), the app is online and a server is configured, it asks `SyncManager` for a silent sync; nothing runs while the app is backgrounded, and a missed interval catches up on the next open. Only the time of the last attempt is persisted, so a server that keeps failing is retried once per interval rather than on every poll. A silent sync still shows the progress notification and the Settings spinner, but never a toast.
 
-Songs have a primary and a fallback fetch. The primary probes `search3` with an empty query once (memoized per source); if the server supports it, the library is paged through `search3` 500 songs at a time. Otherwise the fallback pages `getAlbumList2` and then issues one `getAlbum` per album — it works everywhere but costs a request per album instead of per 500 songs. Every paged loop stops on a short page and is capped at 1000 pages. Playlists are fetched whole, and their songs are stored by position so a playlist that got shorter (or was emptied) is trimmed at the end.
+Songs have a primary and a fallback fetch. The primary probes `search3` with an empty query once (memoized per source); if the server supports it, the library is paged through `search3` 500 songs at a time. Otherwise the fallback pages `getAlbumList2` and then issues one `getAlbum` per album — it works everywhere but costs a request per album instead of per 500 songs, though it still coalesces the songs into 500-row batches before writing them. Every paged loop stops on a short page and is capped at 20,000 pages. Playlists are fetched whole, and their songs are stored by position so a playlist that got shorter (or was emptied) is trimmed at the end.
 
 Cleanup is the "everything I had, minus everything the server just sent" step described above: artists, albums, songs and playlists delete the leftover IDs in 500-row chunks, disc titles are deleted per `(album, disc)` for albums that still exist, and playlist entries beyond the new count are removed along with entries for playlists that disappeared.