Add PlayFab inventory filter endpoint and remove custom entitlements filtering

[Daniel-Ric]

Motivation

• Provide an official server-side way to query PlayFab Economy inventory using PlayFab filter queries instead of relying on fragile local heuristics.
• Remove duplicated client-side subscription detection logic to simplify code and use PlayFab's authoritative filtering.
• Expose a documented endpoint for consumers who need paged, filtered PlayFab inventory payloads.

Description

• Deleted the custom entitlements heuristic (src/utils/entitlements.js) and removed local filter=subscription handling from the Minecraft entitlements endpoints.
• Added a new endpoint GET /inventory/playfab/items in src/routes/inventory.routes.js that requires x-playfab-session and x-playfab-id headers and validates filter, count, and continuationToken inputs.
• Implemented getEntityTokenForPlayer and getInventoryItems in src/services/playfab.service.js to mint an EntityToken for a player and call PlayFab Inventory/GetInventoryItems with optional Filter, Count, and ContinuationToken.
• Updated the OpenAPI spec (src/utils/swagger.js) and the README.md to document the new PlayFab filter endpoint and example usage.