refactor(core): create MediaItem abstract base class (#117)

* feat(db): add MediaType discriminator to Movies table

Adds foundation for multi-media support by:
- Adding MediaType column to Movies table (migration 244)
- Adding MediaType property to Movie entity, defaulting to Movie

Existing movies will have MediaType=1 (Movie) after migration.
This prepares for future Book and Audiobook media types.

Addresses Issue #1

* refactor(core): create MediaItem abstract base class

Extracts common properties from Movie into a new MediaItem base class:
- MediaType, Monitored, QualityProfileId
- Path, RootFolderPath, Added, Tags, LastSearchTime

Movie now inherits from MediaItem and implements abstract methods
GetTitle() and GetYear() while maintaining backward-compatible
Title/Year property accessors.

This prepares for Book and Audiobook entities that will share
the same base structure.

Addresses Issue #1

---------

Co-authored-by: admin <admin@ardentleatherworks.com>

src/NzbDrone.Core/MediaItems/MediaItem.cs

@@ -0,0 +1,27 @@
+using System;
+using System.Collections.Generic;
+using NzbDrone.Core.Datastore;
+using NzbDrone.Core.MediaTypes;
+
+namespace NzbDrone.Core.MediaItems
+{
+    public abstract class MediaItem : ModelBase
+    {
+        protected MediaItem()
+        {
+            Tags = new HashSet<int>();
+        }
+
+        public MediaType MediaType { get; set; }
+        public bool Monitored { get; set; }
+        public int QualityProfileId { get; set; }
+        public string Path { get; set; }
+        public string RootFolderPath { get; set; }
+        public DateTime Added { get; set; }
+        public HashSet<int> Tags { get; set; }
+        public DateTime? LastSearchTime { get; set; }
+
+        public abstract string GetTitle();
+        public abstract int GetYear();
+    }
+}

src/NzbDrone.Core/Movies/Movie.cs

@@ -1,50 +1,49 @@
 using System;
-using System.Collections.Generic;
 using System.Linq;
 using NzbDrone.Common.Extensions;
 using NzbDrone.Core.Datastore;
 using NzbDrone.Core.MediaFiles;
+using NzbDrone.Core.MediaItems;
 using NzbDrone.Core.MediaTypes;
 using NzbDrone.Core.Profiles.Qualities;
 
 namespace NzbDrone.Core.Movies
 {
-    public class Movie : ModelBase
+    public class Movie : MediaItem
     {
         public Movie()
         {
-            Tags = new HashSet<int>();
             MovieMetadata = new MovieMetadata();
             MediaType = MediaType.Movie;
         }
 
         public int MovieMetadataId { get; set; }
-        public MediaType MediaType { get; set; }
-
-        public bool Monitored { get; set; }
         public MovieStatusType MinimumAvailability { get; set; }
-        public int QualityProfileId { get; set; }
-
-        public string Path { get; set; }
 
         public LazyLoaded<MovieMetadata> MovieMetadata { get; set; }
 
-        public string RootFolderPath { get; set; }
-        public DateTime Added { get; set; }
         public QualityProfile QualityProfile { get; set; }
-        public HashSet<int> Tags { get; set; }
         public AddMovieOptions AddOptions { get; set; }
-        public DateTime? LastSearchTime { get; set; }
         public MovieFile MovieFile { get; set; }
         public int MovieFileId { get; set; }
 
         public bool HasFile => MovieFileId > 0;
 
-        // compatibility properties
+        // MediaItem abstract method implementations
+        public override string GetTitle() => MovieMetadata.Value.Title;
+        public override int GetYear() => MovieMetadata.Value.Year;
+
+        // Backward-compatible property accessors
         public string Title
         {
-            get { return MovieMetadata.Value.Title; }
-            set { MovieMetadata.Value.Title = value; }
+            get => MovieMetadata.Value.Title;
+            set => MovieMetadata.Value.Title = value;
+        }
+
+        public int Year
+        {
+            get => MovieMetadata.Value.Year;
+            set => MovieMetadata.Value.Year = value;
         }
 
         public int TmdbId
@@ -59,12 +58,6 @@ public string ImdbId
             set { MovieMetadata.Value.ImdbId = value; }
         }
 
-        public int Year
-        {
-            get { return MovieMetadata.Value.Year; }
-            set { MovieMetadata.Value.Year = value; }
-        }
-
         public string FolderName()
         {
             if (Path.IsNullOrWhiteSpace())