Settings: modules declare their keys, categories and descriptions

A setting's category is picked from hardcoded key prefixes, `Kubernetes` and `HealthChecks` for Monitoring and `Serilog` for Logging, with everything else under Features: `barakoCMS/Features/Settings/UpdateSetting.cs:97-103`. Descriptions are a switch over the same keys at `UpdateSetting.cs:107-111`. The categories are a fixed public enum, `barakoCMS/Models/SystemSetting.cs:13`.

### Why it is too specific

It knows three core subsystems and nothing else. A module's settings (Resend, Umami, Files storage) all land under Features with no description, and a module cannot add a category such as Email or Storage without a core change. It also works against moving Kubernetes monitoring out to a module (#745), since core would still name it.

### The general concept

Modules and core features declare their setting keys with a category and a description. `UpdateSetting` looks the key up there and falls back to Features.

### Where it lives

Core provides the registry, and each module declares its own keys.

### Compatibility

Released. `SettingCategory` is a public enum in `Models`, which is package surface, so its four values stay. The category is already sent as a string (`barakoCMS/Features/Settings/GetSettings.cs:48`), so new categories do not change the HTTP contract. Stored settings keep the category they were saved with. Near #745.

### Done when

- A module that declares `Resend:FromAddress` under Email sees that category and description in `GET /api/settings`, and a test showing that fails before the change.
- `UpdateSetting` has no hardcoded key prefixes or description switch.
- Existing Kubernetes, HealthChecks and Serilog settings keep their categories.

Found in the too specific sweep of 15 September 2026.

Start with barakoCMS/Features/Settings/UpdateSetting.cs:97-111 and the SettingCategory enum in barakoCMS/Models/SystemSetting.cs:13, then inspect module setting declarations and GetSettings.cs:48. Add the registry and module declarations so Resend:FromAddress returns its category and description, remove the hardcoded mappings, and verify existing Kubernetes, HealthChecks, and Serilog categories remain unchanged.

Written by the indexing model from the issue text.