Allow multiple digital stores per game

[mforce]

Problem

A game can currently only be associated with a single digital store (DigitalStore? — one value from Steam, GOG, Epic, Xbox, PSN, Nintendo, Other). In practice many games are owned on multiple stores (e.g. Steam + Epic giveaway, or Steam + GOG). Users have to pick one or create duplicate entries.

Current model

// Domain entity
public bool IsDigital { get; set; }
public DigitalStore? DigitalStore { get; set; }  // single value

// Enum
public enum DigitalStore
{
    Steam = 0, Gog = 1, Epic = 2, Xbox = 3,
    Psn = 4, Nintendo = 5, Other = 99,
}

Frontend: checkbox "Digital copy" + single <Select> dropdown for store (GameForm.tsx:202).

Proposed solution

Replace the single-value DigitalStore? with a collection of stores (flags enum or join table).

Approach A — Flags enum (recommended for MVP)

Change DigitalStore to a [Flags] enum and store as a bitmask on the Game entity:

[Flags]
public enum DigitalStore
{
    None    = 0,
    Steam   = 1,
    Gog     = 2,
    Epic    = 4,
    Xbox    = 8,
    Psn     = 16,
    Nintendo = 32,
    Other   = 64,
}

Entity change:

// Remove IsDigital — derive from DigitalStore != None
public DigitalStore DigitalStores { get; set; } = DigitalStore.None;

Remove IsDigital property; a game is digital when DigitalStores != DigitalStore.None. Physical + digital ownership is already modeled separately (the Platform field covers the physical side).

Pros: Zero schema migration complexity (single int column), no new entity, consistent with how MovieFormat already works in this codebase.

Cons: Limited to ~7 predefined stores before running into bit-position constraints (currently 7 + None = 8 values, fits in a byte). Adding new stores requires an enum update + EF migration if values shift.

Approach B — Join table

New entity GameDigitalStore with GameId + DigitalStore columns. More flexible for future expansion (free-text store names, per-store acquisition dates/prices) but heavier lift.

Recommendation: Start with Approach A (flags enum). If per-store metadata is needed later, migrate to Approach B.

Changes

Backend

File	Change
Domain/Enums/DigitalStore.cs	Add [Flags], renumber as powers of 2, add None = 0
Domain/Entities/Game.cs	Replace IsDigital (bool) + DigitalStore? (enum?) with DigitalStores (flags enum, default None)
Infrastructure/Data/CollectifyDbContext.cs	Update configuration if any
Api/Endpoints/GasesEndpoints.cs	Update DTO, filters, ApplyDto, ToDto — accept DigitalStores as bitmask int. Derive "is digital" from DigitalStores != None for filtering.
Migration	New EF migration for column rename + backfill

Frontend

File	Change
services/types.ts	Replace isDigital: boolean + digitalStore?: DigitalStore with digitalStores: number (bitmask). Add DIGITAL_STORE_FLAGS array mirroring MOVIE_FORMAT_FLAGS pattern.
components/GameForm.tsx	Replace checkbox + single select with toggle buttons (same UX as MOVIE_FORMAT_FLAGS in MovieForm.tsx)
components/DetailView.tsx	Show multiple store pills instead of single "Digital (Steam)" pill
pages/GamesList.tsx	Show store icon(s)/label(s) in tertiary line
components/FiltersPanel.tsx	Update digital store filter to multi-select checkboxes

Data migration

Backfill existing rows: if IsDigital == true and DigitalStore is set, copy to corresponding bit in DigitalStores. If IsDigital == true but DigitalStore is null, set DigitalStores = Other.

Out of scope

• Per-store acquisition date/price (would require join-table approach)
• Digital store icons (tracked in Add style icons for Movie Formats, Music Formats and Game Platforms #90)
• Free-text store names beyond the enum set

Verification

• dotnet build clean, dotnet test passes
• Add a game with Steam + Epic selected → both stored and displayed
• Edit an existing physical-only game, add digital stores → platform preserved
• Filter games by digital store → correct results
• Existing rows survive migration with stores preserved