feat(server): add percentage-based limits to smart playlists (#5144)

* feat(playlists): add percentage-based limits to smart playlists

Add a new `limitPercent` JSON field to Criteria that allows smart playlist
limits to be expressed as a percentage of matching tracks rather than a
fixed number. For example, a playlist matching 450 songs with a 10% limit
returns 45 songs, scaling dynamically as the library grows.

When `limitPercent` is set, refreshSmartPlaylist runs a COUNT query first
to determine the total matching tracks, then resolves the percentage to an
absolute LIMIT before executing the main query. The fixed `limit` field
takes precedence when both are set. Values are clamped to [0, 100] during
JSON unmarshaling.

No database migration is needed since rules are stored as a JSON string.

* fix(criteria): validate percentage limit range in IsPercentageLimit method

Signed-off-by: Deluan <deluan@navidrome.org>

* fix(criteria): ensure idempotency of ToSql method for expressions

Signed-off-by: Deluan <deluan@navidrome.org>

---------

Signed-off-by: Deluan <deluan@navidrome.org>

model/criteria/operators_test.go

@@ -178,6 +178,21 @@ var _ = Describe("Operators", func() {
 		})
 	})
 
+	DescribeTable("ToSql idempotency",
+		func(expr Expression) {
+			sql1, args1, err1 := expr.ToSql()
+			sql2, args2, err2 := expr.ToSql()
+
+			gomega.Expect(err1).ToNot(gomega.HaveOccurred())
+			gomega.Expect(err2).ToNot(gomega.HaveOccurred())
+			gomega.Expect(sql2).To(gomega.Equal(sql1))
+			gomega.Expect(args2).To(gomega.Equal(args1))
+		},
+		Entry("tag expression", Is{"genre": "Rock"}),
+		Entry("role expression", Contains{"artist": "Beatles"}),
+		Entry("nested criteria", Criteria{Expression: All{Is{"genre": "Rock"}, Contains{"artist": "Beatles"}}}),
+	)
+
 	DescribeTable("JSON Marshaling",
 		func(op Expression, jsonString string) {
 			obj := And{op}