feat: add series support for books and games (#54)

Implements series grouping functionality for books and games to allow
tracking franchises and related items.

Database changes:
- Add series (optional string) and seriesOrder (optional number) fields to Book model
- Add series (optional string) and seriesOrder (optional number) fields to Game model

Backend changes:
- Add GET /books/series/:seriesName endpoint to fetch all books in a series
- Add GET /games/series/:seriesName endpoint to fetch all games in a series
- Add getBooksBySeries() method to BookService (orders by seriesOrder asc)
- Add getGamesBySeries() method to GameService (orders by seriesOrder asc)
- Create/Update endpoints automatically handle series fields via DTOs

Frontend changes:
- Add series and seriesOrder input fields to "Add New Book" form
- Add series and seriesOrder input fields to "Edit Book" form
- Add series and seriesOrder input fields to "Add New Game" form
- Add series and seriesOrder input fields to "Edit Game" form

Type definitions:
- Update Book and CreateBookDto interfaces with series fields
- Update Game and CreateGameDto interfaces with series fields

Closes #54

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

shared-types/src/lib/book.types.ts

@@ -27,6 +27,8 @@ interface Book {
   coverImage?:   string;
   tags:          Array<string>;
   links:         Array<Link>;
+  series?:       string;
+  seriesOrder?:  number;
   createdAt:     Date;
   updatedAt:     Date;
 }
@@ -43,6 +45,8 @@ interface CreateBookDto {
   coverImage?:   string;
   tags?:         Array<string>;
   links?:        Array<Link>;
+  series?:       string;
+  seriesOrder?:  number;
 }
 
 interface UpdateBookDto extends Partial<CreateBookDto> {

shared-types/src/lib/game.types.ts

@@ -27,6 +27,8 @@ interface Game {
   coverImage?:    string;
   tags:           Array<string>;
   links:          Array<Link>;
+  series?:        string;
+  seriesOrder?:   number;
   createdAt:      Date;
   updatedAt:      Date;
 }
@@ -42,6 +44,8 @@ interface CreateGameDto {
   coverImage?:   string;
   tags?:         Array<string>;
   links?:        Array<Link>;
+  series?:       string;
+  seriesOrder?:  number;
 }
 
 interface UpdateGameDto extends Partial<CreateGameDto> {