Add PlayFab inventory filter shortcuts

[Daniel-Ric]

Motivation

• PlayFab Inventory/GetInventoryItems only accepts filters on type, id, or stackId using eq, so existing queries using unsupported fields fail; the endpoint needs a safe, convenient way to express these allowed filters.
• Provide a simple, documented shortcut API for common queries (e.g. subscriptions) while keeping full filter support for advanced use.

Description

• Added optional query parameters type, id, and stackId to GET /inventory/playfab/items and validate that callers use either filter or the shortcut params, not both, and require one of them to be present.
• Implemented buildInventoryFilter and escapeFilterValue in src/routes/inventory.routes.js to construct a PlayFab-compatible Filter string and safely escape single quotes.
• Updated OpenAPI schema in src/utils/swagger.js and examples in README.md to document supported filter fields and show both filter and shortcut usage examples.

Testing

• Ran a smoke import test with JWT_SECRET=0123456789abcdef SWAGGER_SERVER_URL=http://localhost node --input-type=module -e "import './src/routes/inventory.routes.js'; console.log('ok');" and it printed ok, indicating the route module loads successfully (pass).
• No unit tests are present in the repository, and no additional automated tests were executed beyond the module import check (n/a for failures).

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