feat(search): 'why this matched' highlights (ts_headline)

Slice 3 of search-core. Search results now show WHY they matched: a ts_headline
body fragment with matched terms highlighted. The API returns U+E000/U+E001
sentinels around matches (NOT HTML); the client tokenises them into escaped
<mark> React elements, so untrusted email content can never inject markup (no
dangerouslySetInnerHTML). Subtle accent tint. Only when there's a free-text query
(browse unchanged); projection branched (no DB function in a ternary) for
unambiguous EF translation.

Verified: build + all 40 tests; ts_headline output shape in raw Postgres; and the
ranked+headline EF query proven to translate + execute against a live Postgres via
a throwaway test (removed).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/components/EmailRow.jsx

@@ -16,6 +16,25 @@ const fmtSize = (b) => {
   return `${(b / 1048576).toFixed(1)} MB`;
 };
 
+// "Why this matched": the API wraps matched terms in U+E000/U+E001 sentinels (NOT HTML).
+// We tokenise and render the highlighted parts as <mark> React elements — React escapes
+// all text nodes, so untrusted email content can never inject markup (no dangerouslySetInnerHTML).
+const HL_START = String.fromCharCode(0xE000);
+const HL_STOP = String.fromCharCode(0xE001);
+const HL_RE = new RegExp(HL_START + '([\s\S]*?)' + HL_STOP, 'g');
+function renderHighlight(s) {
+  const out = [];
+  let last = 0, key = 0, m;
+  HL_RE.lastIndex = 0;
+  while ((m = HL_RE.exec(s)) !== null) {
+    if (m.index > last) out.push(s.slice(last, m.index));
+    out.push(<mark key={key++}>{m[1]}</mark>);
+    last = HL_RE.lastIndex;
+  }
+  if (last < s.length) out.push(s.slice(last));
+  return out;
+}
+
 export default function EmailRow({ email: initial, onRemove, selected, onToggleSelect, focused }) {
   const [email, setEmail] = useState(initial);
   const [acting, setActing] = useState(false);
@@ -83,7 +102,9 @@ export default function EmailRow({ email: initial, onRemove, selected, onToggleS
       </td>
       <td className="el-subject">
         <span className="el-subj-text">{email.subject || '(no subject)'}</span>
-        {email.snippet && <span className="el-snippet"> — {email.snippet}</span>}
+        {email.matchHighlight
+          ? <span className="el-snippet"> — {renderHighlight(email.matchHighlight)}</span>
+          : email.snippet && <span className="el-snippet"> — {email.snippet}</span>}
       </td>
       <td className="el-meta">
         {email.hasAttachments && <span className="el-attach" title="Has attachment">📎</span>}

frontend/src/index.css

@@ -64,6 +64,14 @@
     border-color: hsl(var(--border));
   }
 
+  /* Search "why this matched" highlight — subtle accent tint, not the default yellow. */
+  mark {
+    background: hsl(var(--primary) / 0.22);
+    color: inherit;
+    border-radius: 3px;
+    padding: 0 1px;
+  }
+
   body {
     @apply bg-background text-foreground antialiased;
     /* Inter (variable, self-hosted via @fontsource-variable/inter); system fallback. */

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>
         {