Support filtering prometheusQueryPresets by category name

## Context

The WebUI deployment detail page (FR-3434, lablup/backend.ai-webui#8611) renders vLLM inference metric charts by fetching Prometheus query presets in the `vllm-inference` category and executing them via `prometheusQueryPresetResult`.

## Problem

`prometheusQueryPresets`' `QueryDefinitionFilter` only supports filtering by `categoryId` (UUID). The category UUID is generated by the DB at seed time (`_seed_category` in migration `7af18070fdef`), so it differs per installation and cannot be known by clients upfront. As a result the WebUI must fetch **all** presets and filter client-side by `category.name`, or spend an extra round trip resolving the category UUID first.

Meanwhile the category **name** is the de-facto stable semantic key:

- `prometheus_query_preset_categories.name` has a UNIQUE constraint
- the seed migration upserts idempotently keyed by name
- there is no rename mutation (only `adminCreatePrometheusQueryPresetCategory` / `adminDelete...`)

## Request

Add a category-name filter to `QueryDefinitionFilter`, e.g.:

```
prometheusQueryPresets(filter: { categoryName: { equals: "vllm-inference" } }) { ... }
```

(a `StringFilter` joined through the category relation). This lets clients select a preset category in a single server-side-filtered query using the stable key.

## Notes

- Nice-to-have in the same area: `prometheusQueryPresetCategories` already supports a name filter via `CategoryFilter`, so this aligns the two.
- Reported from WebUI work on FR-3434.

JIRA Issue: BA-7263

Start at the prometheusQueryPresets GraphQL entry point and QueryDefinitionFilter; inspect how categoryId filtering is implemented and how category names are filtered by CategoryFilter. Use the existing category-name filtering behavior as the reference, then verify that categoryName selects only matching presets while existing filters continue to work.

Written by the indexing model from the issue text.