feat: Add PdfOperationsController with dual-input merge endpoints

- Add PdfOperationsController.cs with route '/api/pdf/operations'
- MergeFromFiles: POST /merge (multipart/form-data) - accepts List<IFormFile>
- MergeFromBase64: POST /merge (application/json) - accepts MergePdfsRequest DTO
- Returns merged PDF as FileContentResult (application/pdf)
- Supports optional page ranges via JSON endpoint only (multipart binding complex)
- XML documentation with response codes (200, 400, 500)
- Uses primary constructor pattern

DocumentOperator.API/Controllers/PdfOperationsController.cs

@@ -0,0 +1,120 @@
+using DocumentOperator.Application.MergePdfs;
+using DocumentOperator.Domain.Common.Exceptions;
+using MediatR;
+using Microsoft.AspNetCore.Http;
+using Microsoft.AspNetCore.Mvc;
+
+namespace DocumentOperator.API.Controllers;
+
+/// <summary>
+/// Controller for PDF operations (merge, stamp, annotate).
+/// </summary>
+[ApiController]
+[Route("api/pdf/operations")]
+public class PdfOperationsController(IMediator mediator) : ControllerBase
+{
+    /// <summary>
+    /// Merges multiple PDF files into a single PDF.
+    /// Supports multipart/form-data file upload.
+    /// </summary>
+    /// <param name="files">PDF files to merge (minimum 2 required)</param>
+    /// <param name="cancellationToken">Cancellation token</param>
+    /// <returns>Merged PDF file</returns>
+    /// <response code="200">PDFs merged successfully - returns merged PDF</response>
+    /// <response code="400">Invalid input (fewer than 2 files, corrupted PDF)</response>
+    /// <response code="500">Internal server error during PDF processing</response>
+    [HttpPost("merge")]
+    [Consumes("multipart/form-data")]
+    [ProducesResponseType(typeof(FileContentResult), StatusCodes.Status200OK)]
+    [ProducesResponseType(typeof(ProblemDetails), StatusCodes.Status400BadRequest)]
+    [ProducesResponseType(typeof(ProblemDetails), StatusCodes.Status500InternalServerError)]
+    public async Task<IActionResult> MergeFromFiles(
+        [FromForm] List<IFormFile> files,
+        CancellationToken cancellationToken)
+    {
+        // Convert IFormFile[] to Stream[] (use OpenReadStream directly - no buffering)
+        var streams = files.Select(f => f.OpenReadStream()).ToList();
+
+        // Send command to MediatR (no page ranges for now - multipart binding is complex)
+        var command = new MergePdfsCommand 
+        { 
+            PdfStreams = streams,
+            PageRanges = null
+        };
+        
+        byte[] mergedPdf = await mediator.Send(command, cancellationToken);
+
+        // Return merged PDF
+        return File(mergedPdf, "application/pdf", "merged.pdf");
+    }
+
+    /// <summary>
+    /// Merges multiple PDF files into a single PDF.
+    /// Supports Base64-encoded PDFs via JSON payload.
+    /// </summary>
+    /// <param name="request">Request containing Base64-encoded PDFs and optional page ranges</param>
+    /// <param name="cancellationToken">Cancellation token</param>
+    /// <returns>Merged PDF file</returns>
+    /// <response code="200">PDFs merged successfully - returns merged PDF</response>
+    /// <response code="400">Invalid input (Base64 format error, fewer than 2 files, corrupted PDF, invalid page range)</response>
+    /// <response code="500">Internal server error during PDF processing</response>
+    [HttpPost("merge")]
+    [Consumes("application/json")]
+    [ProducesResponseType(typeof(FileContentResult), StatusCodes.Status200OK)]
+    [ProducesResponseType(typeof(ProblemDetails), StatusCodes.Status400BadRequest)]
+    [ProducesResponseType(typeof(ProblemDetails), StatusCodes.Status500InternalServerError)]
+    public async Task<IActionResult> MergeFromBase64(
+        [FromBody] MergePdfsRequest request,
+        CancellationToken cancellationToken)
+    {
+        // Convert Base64[] to MemoryStream[]
+        List<Stream> streams = new();
+        try
+        {
+            foreach (var base64Pdf in request.Base64Pdfs)
+            {
+                byte[] pdfBytes = Convert.FromBase64String(base64Pdf);
+                streams.Add(new MemoryStream(pdfBytes));
+            }
+        }
+        catch (FormatException ex)
+        {
+            // Dispose opened streams on error
+            foreach (var stream in streams) stream.Dispose();
+            throw new BadRequestException("Invalid Base64 format: " + ex.Message);
+        }
+
+        var command = new MergePdfsCommand 
+        { 
+            PdfStreams = streams,
+            PageRanges = request.PageRanges
+        };
+        
+        byte[] mergedPdf = await mediator.Send(command, cancellationToken);
+
+        // Cleanup streams (important for MemoryStreams we created)
+        foreach (var stream in streams) stream.Dispose();
+
+        return File(mergedPdf, "application/pdf", "merged.pdf");
+    }
+}
+
+/// <summary>
+/// Request DTO for Base64-encoded PDF merge operation
+/// </summary>
+public record MergePdfsRequest
+{
+    /// <summary>
+    /// Array of Base64-encoded PDF files (minimum 2 required)
+    /// </summary>
+    /// <example>["JVBERi0xLjQK...", "JVBERi0xLjQK..."]</example>
+    public List<string> Base64Pdfs { get; init; } = new();
+
+    /// <summary>
+    /// Optional page ranges per PDF (null = all pages). 
+    /// Format: "1-3,5" means pages 1, 2, 3, and 5.
+    /// If provided, array length must match Base64Pdfs length.
+    /// </summary>
+    /// <example>["1-2", "1,3,5", null]</example>
+    public List<string?>? PageRanges { get; init; }
+}