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/ContactController.cs

@@ -115,7 +115,7 @@ namespace Api.Controllers
             catch (Exception ex)
             {
                 _log.LogError(ex, "Subscription failed. ip={Ip} eMail={eMail}", userIp, req.Email);
-                return StatusCode(StatusCodes.Status500InternalServerError, new ErrorResponse { Error = "Failed.", Code = "subscription_failed" });
+                return StatusCode(StatusCodes.Status500InternalServerError, new ErrorResponse { Error = "Could not process subscription.", Code = "subscription_failed" });
             }
 
         }