Files: IFileStore in Core/Interfaces, implemented by the Files module

File storage is only reachable from inside the Files module. `IFileStorage` lives in `BarakoCMS.Files/IFileStorage.cs:11`, and nothing in core references it. Mail already has the other shape: `IEmailService` sits in `barakoCMS/Core/Interfaces/IEmailService.cs` and a module implements it.

### Why it matters

Attaching a stored file to an email (#806), writing a report export (#837), generating a PDF invoice (#585), keeping a payment receipt, and a microservice writing its output all need to store or read bytes. Each would have to reference the Files module directly, so a module that wants a file cannot run without Files installed, or core ends up referencing a module, which CLAUDE.md section 1 rules out.

### The general concept

An `IFileStore` in `Core/Interfaces` covering put, get, delete and a public URL for stored files, implemented by the Files module (and its S3 provider). Anything that needs a file asks core for the interface and gets a clear error when no module provides it.

### Where it lives

Core: the interface. Module: BarakoCMS.Files implements it.

### Compatibility

A new public interface in core, which is a deliberate addition to the package surface. `IFileStorage` in the Files module stays as it is and its implementations also implement the core interface, so no module or host breaks. No HTTP or stored data change.

### Done when

- A module with no reference to BarakoCMS.Files stores and reads a file through the core interface in a test.
- Without the Files module installed, resolving the interface gives a message naming the missing module.
- #806 can be built against the core interface.

Found in the architecture sweep of 15 September 2026.

Start with BarakoCMS.Files/IFileStorage.cs:11, barakoCMS/Core/Interfaces/IEmailService.cs, and CLAUDE.md section 1 to understand the existing interface boundary. Trace the Files module and its S3 provider, then add tests showing a module without a Files reference can store and read through the core interface, missing-module resolution names the module, and #806 can use it.

Written by the indexing model from the issue text.