feat(search): fuzzy/typo tolerance fallback (pg_trgm word_similarity)

Slice 4 of search-core: when an exact free-text search returns zero results, fall
back to pg_trgm word_similarity on the subject (explicit 0.3 threshold — pg_trgm's
default 0.6 misses real typos like 'recieved'->'received' ~0.39). Fallback-only, so
it never dilutes good exact matches; fuzzy hits get no ts_headline. Migration enables
the pg_trgm extension (CREATE EXTENSION IF NOT EXISTS — idempotent).

Also relocates the EmailSummaryDto.MatchHighlight note to an XML doc: a comment inside
the record's parameter list was tripping dotnet-format's whitespace check on every
commit (mutating format wouldn't fix it) — moving it out clears that drift for good.

Verified end-to-end vs a live Postgres via throwaway test (removed): typo 'recieved'
fuzzy-matched a subject with 'received'. Build + all 40 tests pass.

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

src/InboxIntel.Infrastructure/Migrations/20260701203001_EnablePgTrgm.cs

@@ -0,0 +1,24 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace InboxIntel.Infrastructure.Migrations
+{
+    /// <inheritdoc />
+    public partial class EnablePgTrgm : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.AlterDatabase()
+                .Annotation("Npgsql:PostgresExtension:pg_trgm", ",,");
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.AlterDatabase()
+                .OldAnnotation("Npgsql:PostgresExtension:pg_trgm", ",,");
+        }
+    }
+}