Fix Spotify token/account joins and add guards

Join spotifyAccounts to spotifyTokens by userId (was using the wrong app
id), and add guard clauses that warn and return undefined when no tokens
or when token/app data are missing to avoid runtime errors.

apps/api/src/xrpc/app/rocksky/song/matchSong.ts

···
     .from(tables.spotifyTokens)
     .leftJoin(
       tables.spotifyApps,
-      eq(tables.spotifyTokens.spotifyAppId, tables.spotifyApps.spotifyAppId),
+      eq(tables.spotifyApps.spotifyAppId, tables.spotifyTokens.spotifyAppId),
     )
     .leftJoin(
       tables.spotifyAccounts,
-      eq(tables.spotifyAccounts.spotifyAppId, tables.spotifyApps.id),
+      eq(tables.spotifyAccounts.userId, tables.spotifyTokens.userId),
     )
     .where(eq(tables.spotifyAccounts.isBetaUser, true))
     .limit(500)
     .execute();
 
+  if (!spotifyTokens || spotifyTokens.length === 0) {
+    console.warn("No Spotify tokens available for beta users");
+    return undefined;
+  }
+
   const { spotify_tokens, spotify_apps } =
     spotifyTokens[Math.floor(Math.random() * spotifyTokens.length)];
+
+  if (!spotify_tokens || !spotify_apps) {
+    console.warn("Invalid Spotify token or app data");
+    return undefined;
+  }
 
   const refreshToken = decrypt(
     spotify_tokens.refreshToken,