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>

api/prisma/schema.prisma

@@ -32,6 +32,8 @@ model Game {
   coverImage   String?
   tags         String[]
   links        Link[]
+  series       String?
+  seriesOrder  Int?      @db.Int
   createdAt    DateTime  @default(now())
   updatedAt    DateTime  @updatedAt
   comments     Comment[]
@@ -58,6 +60,8 @@ model Book {
   coverImage   String?
   tags         String[]
   links        Link[]
+  series       String?
+  seriesOrder  Int?      @db.Int
   createdAt    DateTime  @default(now())
   updatedAt    DateTime  @updatedAt
   comments     Comment[]