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>

apps/frontend/src/app/components/books/books-list.component.ts

@@ -127,6 +127,29 @@ import { Book, BookStatus, CreateBookDto, UpdateBookDto, Comment, SuggestionEnti
             ></textarea>
           </div>
 
+          <div class="form-group">
+            <label for="series">Series (optional)</label>
+            <input
+              type="text"
+              id="series"
+              [(ngModel)]="newBook.series"
+              name="series"
+              placeholder="e.g., Harry Potter"
+            >
+          </div>
+
+          <div class="form-group">
+            <label for="seriesOrder">Series Order (optional)</label>
+            <input
+              type="number"
+              id="seriesOrder"
+              [(ngModel)]="newBook.seriesOrder"
+              name="seriesOrder"
+              min="1"
+              placeholder="Order in series"
+            >
+          </div>
+
           <div class="form-group">
             <label for="coverImage">Cover Image (max 500KB)</label>
             <input
@@ -289,6 +312,29 @@ import { Book, BookStatus, CreateBookDto, UpdateBookDto, Comment, SuggestionEnti
             ></textarea>
           </div>
 
+          <div class="form-group">
+            <label for="edit-series">Series (optional)</label>
+            <input
+              type="text"
+              id="edit-series"
+              [(ngModel)]="editBook.series"
+              name="series"
+              placeholder="e.g., Harry Potter"
+            >
+          </div>
+
+          <div class="form-group">
+            <label for="edit-seriesOrder">Series Order (optional)</label>
+            <input
+              type="number"
+              id="edit-seriesOrder"
+              [(ngModel)]="editBook.seriesOrder"
+              name="seriesOrder"
+              min="1"
+              placeholder="Order in series"
+            >
+          </div>
+
           <div class="form-group">
             <label for="edit-coverImage">Cover Image (max 500KB)</label>
             <input