feat: add library scanning mechanic

Tools/Seeder.cs

@@ -177,4 +177,41 @@ public class Seeder
 		return migrationSuccess;
 	}
 
+	/// <summary>
+	/// Check if the library needs a full rescan
+	/// </summary>
+	/// <returns>True if full rescan is needed</returns>
+	/// <exception cref="MissingFieldException">Thrown when either last library state, path to music library or cache is unknown</exception>
+	public async Task<bool> ScanPrefetchAsync()
+	{
+		bool scanNecessary = false;
+		Global? lastLibraryState = await db.Globals.FirstOrDefaultAsync(g => g.Key ==       "libraryState");
+		Global?      libraryPath = await db.Globals.FirstOrDefaultAsync(g => g.Key ==   "musicLibraryPath");
+		Global?        cachePath = await db.Globals.FirstOrDefaultAsync(g => g.Key == "musicThumbnailPath");
+
+		if (libraryPath is null || cachePath is null || lastLibraryState is null)
+		{
+			throw new MissingFieldException("[Error] Missing libraryState, musicLibraryPath, musicThumbnailPath. Please rerun the setup wizard with `Shadow setupWizard`.");
+		}
+		LibraryWatcher lw = new(libraryPath.Value!, [cachePath.Value!], db);
+
+		// Get library contents
+		List<string> currentLibraryStateList = await lw.GetAllMultimediaAsync();
+		// Compute their hash
+		string currentLibraryStateString = string.Join("\n", currentLibraryStateList);
+		string currentLibraryState = MetadataExtractor.GetStringMD5(currentLibraryStateString);
+
+		// Compare against last known library state
+		if (currentLibraryState != lastLibraryState.Value)
+			scanNecessary = true;
+
+		// The contents changed? Initiate a full rescan, then call LibraryWatcher.
+		if (scanNecessary)
+			await lw.PerformFullScanAsync();
+		// State seems identical? Launch just the LibraryWatcher.
+		// TODO: lw.Watch()...
+
+		return scanNecessary;
+
+	}
 }