Implement Disc entity with complete CRUD operations

Add full implementation for Disc entity following existing patterns:
  - Create Disc model with Album relationship
  - Add DiscRepository for data persistence
  - Implement DiscService with business logic
  - Create DiscMapper for protobuf conversion
  - Add all handler classes (Create, Get, GetById, Update, Delete)
  - Register DiscService in DelegateActionManager
  - Define disc.proto with all message types

  🤖 Generated with [Claude Code](https://claude.com/claude-code)

  Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/main/proto/disc.proto

@@ -0,0 +1,54 @@
+syntax = "proto3";
+
+option java_package = "com.mediamanager.protocol.messages";
+option java_outer_classname = "DiscMessages";
+
+package mediamanager.messages;
+
+message Disc{
+  int32 id = 1;
+  int32 disc_number = 2;
+  int32 fk_album_id = 3;
+}
+
+message CreateDiscRequest {
+  int32 disc_number = 1;
+  int32 fk_album_id = 2;
+}
+
+message CreateDiscResponse {
+  Disc disc = 1;
+}
+
+message GetDiscsRequest {
+}
+
+message GetDiscsResponse {
+  repeated Disc discs = 1;
+}
+
+message GetDiscByIdRequest {
+  int32 id = 1;
+}
+
+message GetDiscByIdResponse {
+  Disc disc = 1;
+}
+
+message UpdateDiscRequest {
+  int32 id = 1;
+  int32 disc_number = 2;
+  int32 fk_album_id = 3;
+}
+
+message UpdateDiscResponse {
+  Disc disc = 1;
+}
+
+message DeleteDiscRequest {
+  int32 id = 1;
+}
+
+message DeleteDiscResponse {
+  string message = 1;
+}