Rebrand project: DocumentOperator to DocumentService

This commit implements a complete rebranding of the project:
- Updated all namespaces from `DocumentOperator` to `DocumentService`.
- Renamed file paths, embedded resources, and test data references.
- Updated configuration keys, logging paths, and Redis instance names.
- Revised documentation to reflect the new project name.
- Modified project and solution files to align with the new structure.
- Updated class names, DTOs, commands, queries, and handlers.
- Adjusted middleware, controllers, and API endpoints.
- Updated Swagger metadata and API titles to `DocumentService API`.
- Refactored test namespaces, resource paths, and embedded resources.
- Updated build and deployment configurations for the new name.
- Replaced all references to `DocumentOperator` in comments and literals.

These changes ensure consistency across the codebase and documentation.

AGENTS.md

@@ -1,6 +1,6 @@
 # AGENTS.md
 
-Agent guidance for DocumentOperator service. Read this before working on the codebase.
+Agent guidance for DocumentService service. Read this before working on the codebase.
 
 ---
 
@@ -23,7 +23,7 @@ Agent guidance for DocumentOperator service. Read this before working on the cod
 ### Migration Required
 
 **Existing code that needs replacement:**
-- `DocumentOperator.API/Endpoints/v1/DocumentEndpoints.cs` → Delete, replace with Controllers
+- `DocumentService.API/Endpoints/v1/DocumentEndpoints.cs` → Delete, replace with Controllers
 - `Program.cs` line 72: `app.MapDocumentEndpoints()` → Replace with `app.MapControllers()`
 - `Program.cs` line 44: Add `builder.Services.AddControllers()`
 - All DTOs → Support **BOTH** `IFormFile` (multipart) AND `Base64String` (JSON)
@@ -195,7 +195,7 @@ dotnet test
 
 **Run API (Development):**
 ```powershell
-dotnet run --project DocumentOperator.API
+dotnet run --project DocumentService.API
 ```
 Swagger UI: `https://localhost:7186/swagger`  
 Serilog UI: `https://localhost:7186/serilog-ui` (Web-based log viewer)
@@ -339,7 +339,7 @@ Do NOT skip steps. Each feature is done when it's **testable in Swagger UI with
 **All test PDFs are EmbeddedResource.** Access via:
 ```csharp
 var stream = Assembly.GetExecutingAssembly()
-    .GetManifestResourceStream("DocumentOperator.Tests.TestData.Pdfs.valid.pdf");
+    .GetManifestResourceStream("DocumentService.Tests.TestData.Pdfs.valid.pdf");
 ```
 
 **Do NOT commit new binary files** without marking them as `<EmbeddedResource>`.
@@ -351,7 +351,7 @@ var stream = Assembly.GetExecutingAssembly()
 **3-folder structure (CORRECT approach by previous developer):**
 
 ```
-DocumentOperator.Tests/
+DocumentService.Tests/
 ├── Integration/
 │   └── API/
 │       ├── PdfValidationControllerTests.cs (13 tests)
@@ -520,7 +520,7 @@ public async Task<IActionResult> Validate([FromForm] PdfInputModel input, Cancel
 ## Configuration
 
 **appsettings.json sections:**
-- `DocumentOperatorSettings` (future: file size limits, temp paths)
+- `DocumentServiceSettings` (future: file size limits, temp paths)
 - `RedisSettings` (future: multi-tenancy caching)
 - `ApiKeySettings` (future: authentication)