feat(search): why this matched highlights (#15)

src/InboxIntel.Application/DTOs/EmailDtos.cs

@@ -16,7 +16,12 @@ public record EmailSummaryDto(
     long SizeEstimateBytes,
     EmailCategory Category,
     bool HasListUnsubscribe,
-    bool SupportsOneClick);
+    bool SupportsOneClick,
+    // "Why this matched": a ts_headline fragment of the body with matched terms wrapped in
+    // U+E000/U+E001 sentinels (NOT HTML — the client renders them as escaped <mark> elements,
+    // so untrusted email content can never inject markup). Null unless the search had a
+    // free-text query. Optional/last so other DTO constructors are unaffected.
+    string? MatchHighlight = null);
 
 /// <summary>Full single-email view, including body text, for the detail pane.</summary>
 public record EmailDetailDto(

src/InboxIntel.Infrastructure/Search/SearchService.cs

@@ -77,15 +77,39 @@ public class SearchService : ISearchService
                .ThenByDescending(e => e.SentAtUtc)
             : q.OrderByDescending(e => e.SentAtUtc);
 
-        var items = await ranked
-            .Skip((r.Page - 1) * r.PageSize)
-            .Take(r.PageSize)
-            .Select(e => new EmailSummaryDto(
-                e.Id, e.GmailMessageId, e.Subject, e.Snippet,
-                e.Sender!.Address, e.Sender.DisplayName, e.SentAtUtc,
-                e.IsUnread, e.IsStarred, e.HasAttachments, e.SizeEstimateBytes, e.Category,
-                e.HasListUnsubscribe, e.SupportsOneClickUnsubscribe))
-            .ToListAsync(ct);
+        var paged = ranked.Skip((r.Page - 1) * r.PageSize).Take(r.PageSize);
+
+        // Two unconditional projections (no DB function inside a C# ternary → no doubt about
+        // EF translation). The browse path never touches ts_headline, so it's byte-for-byte
+        // unchanged AND safe under the InMemory test provider.
+        List<EmailSummaryDto> items;
+        if (hasFreeTextQuery)
+        {
+            // "Why this matched": ts_headline body fragment with matched terms wrapped in
+            // U+E000/U+E001 sentinels (safe, non-HTML — the client renders them as escaped
+            // <mark> spans; see EmailSummaryDto).
+            var headlineOpts =
+                $"StartSel={(char)0xE000},StopSel={(char)0xE001},MaxWords=16,MinWords=5,ShortWord=2,HighlightAll=false";
+            items = await paged
+                .Select(e => new EmailSummaryDto(
+                    e.Id, e.GmailMessageId, e.Subject, e.Snippet,
+                    e.Sender!.Address, e.Sender.DisplayName, e.SentAtUtc,
+                    e.IsUnread, e.IsStarred, e.HasAttachments, e.SizeEstimateBytes, e.Category,
+                    e.HasListUnsubscribe, e.SupportsOneClickUnsubscribe,
+                    EF.Functions.WebSearchToTsQuery("english", term).GetResultHeadline("english", e.BodyText ?? "", headlineOpts)))
+                .ToListAsync(ct);
+        }
+        else
+        {
+            items = await paged
+                .Select(e => new EmailSummaryDto(
+                    e.Id, e.GmailMessageId, e.Subject, e.Snippet,
+                    e.Sender!.Address, e.Sender.DisplayName, e.SentAtUtc,
+                    e.IsUnread, e.IsStarred, e.HasAttachments, e.SizeEstimateBytes, e.Category,
+                    e.HasListUnsubscribe, e.SupportsOneClickUnsubscribe,
+                    null))
+                .ToListAsync(ct);
+        }
 
         return new PagedResult<EmailSummaryDto>
         {