address arch review: guard the page cap, coalesce syncs, tighten buffers

- Paged loops now throw when they hit maxPages with a full last page, so a
  truncated enumeration aborts the entity's collect and skips its prune
  instead of deleting rows the sync never saw. The cap is injectable for
  tests.
- Move sync serialization into SyncManager (tryLock coalesces duplicate
  requests) and construct SyncService there; SourceRepository only exposes
  the active MusicSource. Removes the repo <-> sync import cycle.
- Re-throw CancellationException instead of reporting a cancelled sync as
  failed.
- Add buffer(1) to the outer albums/songs/playlistSongs flows so their
  flowOn channel cannot hold 64 batches.
- Rank prefetch is best-effort (missing frequent/recent no longer aborts
  the album sync).
- Skip playlist entries without an id, keeping positions contiguous.
- Chunk stale-disc deletes like the other prunes.
- Update the implementation plan line that still described the old
  single-transaction sync.

app/src/test/kotlin/com/subtracks/data/source/subsonic/SubsonicSourceTest.kt

@@ -179,6 +179,49 @@ class SubsonicSourceTest {
             assertEquals(500, firstBatch.size)
         }
 
+    @Test
+    fun albumsAbortWhenThePageCapIsHit() =
+        runBlocking {
+            server.dispatcher =
+                object : Dispatcher() {
+                    override fun dispatch(request: RecordedRequest): MockResponse {
+                        val url = request.requestUrl!!
+                        if (url.encodedPath != "/rest/getAlbumList2.view") return MockResponse().setResponseCode(404)
+                        return if (url.queryParameter("type") == "newest") {
+                            MockResponse().setBody(albumPage(0, 500))
+                        } else {
+                            MockResponse().setBody(emptyAlbumList())
+                        }
+                    }
+                }
+
+            val failure =
+                runCatching {
+                    SubsonicSource(1, client(), maxPages = 2).albums().toList()
+                }
+
+            assertEquals(true, failure.exceptionOrNull() is SubsonicException)
+        }
+
+    @Test
+    fun playlistEntriesWithoutIdsAreSkipped() =
+        runBlocking {
+            server.enqueue(
+                MockResponse().setBody(
+                    "<subsonic-response status=\"ok\"><playlist id=\"p1\">" +
+                        "<entry id=\"s1\" title=\"One\"/>" +
+                        "<entry title=\"No id\"/>" +
+                        "<entry id=\"s2\" title=\"Two\"/>" +
+                        "</playlist></subsonic-response>",
+                ),
+            )
+
+            val entries = SubsonicSource(1, client()).playlistSongs(listOf("p1")).toList().flatten()
+
+            assertEquals(listOf("s1", "s2"), entries.map { it.songId })
+            assertEquals(listOf(0L, 1L), entries.map { it.position })
+        }
+
     @Test
     fun streamUriRequestsTranscodingWhenABitrateIsSet() {
         val uri = SubsonicSource(1, client(), maxBitrate = 128).streamUri("s1").toString()

app/src/test/kotlin/com/subtracks/data/sync/SyncManagerTest.kt

@@ -37,7 +37,7 @@ class SyncManagerTest {
                 .build()
         sourceRepository = SourceRepository(db, OkHttpClient(), fakeUserPreferences())
         queueRepository = QueueRepository(db)
-        manager = SyncManager(sourceRepository, queueRepository)
+        manager = SyncManager(db, sourceRepository, queueRepository)
     }
 
     @After