feat(library): remove a track from the library without deleting the file

RemoveFromLibrary deletes the audio_files rows the way the scan's own
orphan cleanup does and records each path in excluded_paths. The
exclusion is not an enhancement: without it the next scan finds the
file, sees no row and imports it again, so the button undoes itself.

The soft scan compares files on disk against rows in the database, so
surveyAudioFiles and countAudioFiles both take the exclusion set —
otherwise an excluded path makes the two disagree forever and queues a
full scan on every launch. Deleting a row cascades to queue_tracks, so
the removal calls the same CompactQueue hook RemoveLibrary does.

Also lands the requested badge: library-status-indicator is a button
again where it can act, utils/library-status.ts states once what owning
and wanting mean, and the long-declared queued state finally has a
producer.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UDCbcCZQepnpSQYJ6SxxZm

backend/datamap/datamap.go

@@ -193,6 +193,14 @@ var tables = []Table{
 		Note: "Build metadata for explore_index: dump version, coverage " +
 			"tiers, last refresh.",
 	},
+	{
+		Name: "excluded_paths", Kind: Authored, Lifetime: Cascade,
+		Note: "Paths the user removed from the library, which the scanner " +
+			"must not import again. Authored: it is a decision, not " +
+			"derivable from disk. Cascades with its library, and a full " +
+			"rescan clears it \u2014 the only way back for a path removed by " +
+			"mistake.",
+	},
 	{
 		Name: "file_types", Kind: Derived, Lifetime: Retained,
 		Note: "Static lookup rows seeded from code, not user data.",

backend/datamap/datamap_test.go

@@ -240,6 +240,14 @@ func TestAuthoredCascadesAreDeliberate(t *testing.T) {
 		// unsubscribing from an artist must stop the albums it queued
 		// on the user's behalf.
 		"download_requests": true,
+
+		// An exclusion says "do not import this path into library 3".
+		// Remove that library and no scan will ever visit the path
+		// again, so the row has nothing left to exclude it from — and
+		// the data it protects is the *absence* of a row, which the
+		// library removal has already achieved for everything.  Adding
+		// the library back is the user asking to import it afresh.
+		"excluded_paths": true,
 	}
 
 	for _, entry := range datamap.ByKind(datamap.Authored) {