Improve comments and Swagger annotations across services (#26)

- EmailController: add class summary, full SwaggerResponse/ProducesResponseType
  for 400 and 500, and Description on SwaggerOperation
- ContactController: fix terse "Failed." error message to
  "Could not process subscription."
- FileDownloadController: remove redundant XML <response code> tags from
  the public action doc block; convert private-method /// <summary> to //
  (project convention: no XML doc on internal code)
- CvMatcherService: remove two dead commented-out blocks (old email send
  and BuildEmailBody helper)
- JobTokenService: comment the phone/contact-line regex filter in
  ExtractKeywords
- DocumentClassifier: comment the keyword-frequency scoring approach and
  the confidence formula
- TextChunker: comment the sliding-window step (chunkSize - overlap)
- CvSearchJobTask: comment the GdprConsent = true rationale and the
  BuildCvFileName sanitisation logic
- HtmlJobSearcher: comment GetLeftPart(UriPartial.Path) query-strip dedup

Closes #26

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Apis/api/Controllers/FileDownloadController.cs

@@ -44,10 +44,6 @@ namespace Api.Controllers
         /// </summary>
         /// <param name="fileName">The name of the file to download (optional - uses default from settings if not provided)</param>
         /// <returns>File stream with appropriate headers for resumable downloads</returns>
-        /// <response code="200">Full file content</response>
-        /// <response code="206">Partial file content (range request)</response>
-        /// <response code="404">File not found</response>
-        /// <response code="416">Requested range not satisfiable</response>
         [HttpGet("{fileName?}")]
         [SwaggerOperation(Summary = "Download file", Description = "Downloads a file with support for full and ranged (resumable) transfers.")]
         [SwaggerResponse(StatusCodes.Status200OK, "Full file content returned")]
@@ -135,9 +131,7 @@ namespace Api.Controllers
             }
         }
 
-        /// <summary>
-        /// Handles HTTP range requests for partial content downloads and resume support.
-        /// </summary>
+        // Handles HTTP range requests for partial content downloads and resume support.
         private async Task<IActionResult> HandleRangeRequest(
             string filePath,
             long fileLength,
@@ -194,9 +188,7 @@ namespace Api.Controllers
             }
         }
 
-        /// <summary>
-        /// Efficiently streams a specific byte range from source to destination.
-        /// </summary>
+        // Efficiently streams a specific byte range from source to destination.
         private static async Task StreamRangeAsync(Stream source, Stream destination, long bytesToRead)
         {
             var buffer = new byte[BufferSize];