Merge branch 'fix/249-unbounded-growth' into batch/248-249

Both branches add to the same two registries, so the conflicts are
between the two fixes rather than with main:

- backend/app.go: both register a janitor job.  Both are registered.
- backend/maintenance/sweeps.go: both append a job at the end of the
  file.  Both are kept, each with its own closing tail.
- backend/maintenance/maintenance_test.go: both append a test.  Both are
  kept as separate functions.
- backend/library/library.go: #249's orphan-path lyrics delete was
  written against the loop variable before #250 renamed it, so its
  `audioFile.ID` no longer exists in that function.  Adapted to `f.ID`.

Closes #248
Closes #249

backend/maintenance/maintenance_test.go

@@ -734,3 +734,52 @@ func TestStaleArtistMetadataJob(t *testing.T) {
 		}
 	}
 }
+
+// TestStaleSearchClicksJob deletes only the clicks old enough to have
+// left the retention window (#249).
+func TestStaleSearchClicksJob(t *testing.T) {
+	t.Parallel()
+
+	db := database.NewTestDB(t)
+
+	count := func(mbid string) int {
+		t.Helper()
+
+		var n int
+		if err := db.QueryRowWriter(
+			"SELECT COUNT(*) FROM search_clicks WHERE entity_mbid = ?", mbid,
+		).Scan(&n); err != nil {
+			t.Fatalf("count %s: %v", mbid, err)
+		}
+
+		return n
+	}
+
+	seed := func(query, mbid, lastClicked string) {
+		t.Helper()
+
+		if _, err := db.ExecContext(
+			`INSERT INTO search_clicks
+			   (query, entity_mbid, entity_type, click_count, last_clicked)
+			 VALUES (?, ?, 'recording', 1, ?)`,
+			query, mbid, lastClicked,
+		); err != nil {
+			t.Fatalf("seed search_clicks: %v", err)
+		}
+	}
+
+	seed("tide", "aaaa", "2024-01-01 00:00:00") // stale
+	seed("tide", "bbbb", "2999-01-01 00:00:00") // recent
+
+	if _, err := StaleSearchClicksJob(db).Run(context.Background()); err != nil {
+		t.Fatalf("run job: %v", err)
+	}
+
+	if n := count("bbbb"); n != 1 {
+		t.Errorf("recent click was deleted: %d rows, want 1", n)
+	}
+
+	if n := count("aaaa"); n != 0 {
+		t.Errorf("stale click survived: %d rows, want 0", n)
+	}
+}

backend/maintenance/sweeps.go

@@ -662,3 +662,35 @@ func StaleArtistMetadataJob(db *database.DB) Job {
 		},
 	}
 }
+
+// searchClicksRetention is how long a search-click ranking signal stays
+// useful.  search_clicks is authored behavioural data — nothing that
+// owns a row ever drops it — so age is the ceiling that keeps the table
+// from growing without bound for the life of the install (#249).
+const searchClicksRetention = "-180 days"
+
+// StaleSearchClicksJob deletes search-click ranking rows older than the
+// retention window.  Rows are small and the table grows slowly, so this
+// runs daily and does almost nothing most runs.
+func StaleSearchClicksJob(db *database.DB) Job {
+	return Job{
+		Name:        "search-clicks-sweep",
+		MinInterval: dailyInterval,
+		Run: func(_ context.Context) (Result, error) {
+			res, err := db.ExecContext(
+				`DELETE FROM search_clicks
+				 WHERE last_clicked < datetime('now', ?)`,
+				searchClicksRetention,
+			)
+			if err != nil {
+				return Result{}, fmt.Errorf(
+					"delete stale search_clicks rows: %w", err,
+				)
+			}
+
+			rows, _ := res.RowsAffected()
+
+			return Result{RowsDeleted: rows}, nil
+		},
+	}
+}