Customizable placeholder texts for the create vulnerability and audit forms

### Current Behavior

Organizations often have internal conventions for how vulnerability descriptions, recommendations, and audit analyses should be written (what to include, which questions to answer, which process steps to follow). The create vulnerability form and the audit form currently offer no way to surface such guidance - the text fields are empty, and teams have to maintain this guidance in external documents that nobody has open while filling in the forms.

### Proposed Behavior

Add admin-configurable placeholder texts, shown as native input placeholders:

- On the create vulnerability form: description, detail, recommendation, and references fields.
- On the audit form: the comment field and an instruction text for the analysis details field.
- Managed via GET/PUT /v1/customization/text-placeholders; all values stored as standard config properties.
- Off by default; when disabled, all forms are unchanged.

I have a working implementation (backend + frontend) ready to submit as two pull requests.

### Checklist

- [x] I have read and understand the [contributing guidelines](https://github.com/DependencyTrack/dependency-track/blob/main/CONTRIBUTING.md#filing-issues)
- [x] I have checked the [existing issues](https://github.com/DependencyTrack/dependency-track/issues) for whether this enhancement was already requested

Start by reviewing the create vulnerability and audit forms, then trace the GET/PUT /v1/customization/text-placeholders endpoint and its standard config properties. Done means administrators can configure the listed placeholders, the guidance appears in the relevant fields when enabled, and all forms remain unchanged when disabled.

Written by the indexing model from the issue text.