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/rag-api/Services/DocumentClassifier.cs

@@ -24,6 +24,8 @@ public sealed class DocumentClassifier : IDocumentClassifier
             });
         }
 
+        // Keyword-frequency heuristic: count how many characteristic terms each document
+        // type contributes to the text, then pick the type with the highest hit count.
         var lower = text.ToLowerInvariant();
         var scores = new Dictionary<string, int>(StringComparer.OrdinalIgnoreCase)
         {
@@ -37,6 +39,8 @@ public sealed class DocumentClassifier : IDocumentClassifier
 
         var best = scores.OrderByDescending(x => x.Value).First();
         var type = best.Value <= 0 ? "unknown" : best.Key;
+        // Confidence baseline 0.45 + 0.08 per matched keyword term, capped at 0.95.
+        // Zero hits → 0.25 (effectively unknown).
         var confidence = best.Value <= 0 ? 0.25 : Math.Min(0.95, 0.45 + best.Value * 0.08);
 
         return Task.FromResult(new DocumentClassification