fix: preserve album track order for NULL discNumber (nulls first)

Prisma's default ASC sort uses Postgres NULLS LAST, so the compound
order `[discNumber asc, trackNo asc]` added in PR #170 reorders
tracks in mixed-state libraries: any album where some tracks have
been rescanned post-migration (getting discNumber) and others have
not (staying NULL) ends up with the numbered-disc tracks first and
the NULL-disc tracks dumped at the end, breaking the original
trackNo order that users expected.

Fix: use `{ sort: 'asc', nulls: 'first' }` on discNumber in all six
orderBy call sites (library/albums, offline, share, and the three
places in subsonic/library). This keeps NULL-disc tracks sorting
before disc-1 tracks, preserving pre-migration behavior for
all-NULL albums (the common case right after upgrade) and only
slightly reordering partially-rescanned multi-disc albums (rare,
and only until a full rescan completes).

Found during pre-release review.

backend/src/routes/library/albums.ts

@@ -119,7 +119,7 @@ router.get("/albums/:id", async (req, res) => {
         },
         tracks: {
           orderBy: [
-            { discNumber: Prisma.SortOrder.asc },
+            { discNumber: { sort: Prisma.SortOrder.asc, nulls: "first" } },
             { trackNo: Prisma.SortOrder.asc },
           ],
         },

backend/src/routes/offline.ts

@@ -34,7 +34,7 @@ router.post("/albums/:id/download", async (req, res) => {
             include: {
                 tracks: {
                     orderBy: [
-                        { discNumber: "asc" },
+                        { discNumber: { sort: "asc", nulls: "first" } },
                         { trackNo: "asc" },
                     ],
                 },

backend/src/routes/share.ts

@@ -208,7 +208,7 @@ router.get("/:token", shareResolveLimiter, async (req: Request, res: Response) =
                     artist: { select: { id: true, name: true } },
                     tracks: {
                         orderBy: [
-                            { discNumber: "asc" },
+                            { discNumber: { sort: "asc", nulls: "first" } },
                             { trackNo: "asc" },
                         ],
                         select: {

backend/src/routes/subsonic/library.ts

@@ -135,7 +135,7 @@ libraryRouter.all("/getMusicDirectory.view", wrap(async (req, res) => {
             tracks: {
                 where: { corrupt: false },
                 orderBy: [
-                    { discNumber: "asc" },
+                    { discNumber: { sort: "asc", nulls: "first" } },
                     { trackNo: "asc" },
                 ],
             },
@@ -283,7 +283,7 @@ libraryRouter.all("/getMusicDirectory.view", wrap(async (req, res) => {
             artist: { select: { id: true, name: true, displayName: true, genres: true, userGenres: true } },
             tracks: {
                 orderBy: [
-                    { discNumber: "asc" },
+                    { discNumber: { sort: "asc", nulls: "first" } },
                     { trackNo: "asc" },
                 ],
             },
@@ -325,7 +325,7 @@ libraryRouter.all("/getAlbum.view", wrap(async (req, res) => {
             artist: { select: { id: true, name: true, displayName: true, genres: true, userGenres: true } },
             tracks: {
                 orderBy: [
-                    { discNumber: "asc" },
+                    { discNumber: { sort: "asc", nulls: "first" } },
                     { trackNo: "asc" },
                 ],
             },