feat(search): relevance ranking + websearch_to_tsquery

Slice 1 of search-core (docs/discovery/05, MVP): replace date-only ordering with
ts_rank_cd relevance ranking (Npgsql RankCoverDensity) when a free-text query is
present, recency as a tiebreaker; browse-only requests (no query) keep today's
date-descending order unchanged. Upgrade plainto_tsquery -> websearch_to_tsquery
so quotes ("exact phrase"), OR, and -exclusions work as users already expect
from web search boxes.

Verified: ranking SQL semantics (plain/phrase/exclusion/ts_rank_cd score) checked
directly against a live Postgres instance, not just Npgsql's docs. Added a
regression test locking the date-order fallback path (the one behaviour testable
without a live Postgres, matching this repo's existing no-Testcontainers
convention -- see AuthEndpointsTests.TestAppFactory). All 40 tests pass; full
Release build clean.

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

src/InboxIntel.Infrastructure/Search/SearchService.cs

@@ -10,7 +10,9 @@ namespace InboxIntel.Infrastructure.Search;
 /// <summary>
 /// Structured + full-text search. Structured filters compose as SQL WHERE
 /// clauses; free text uses PostgreSQL FTS via the generated SearchVector column
-/// (EF.Functions.ToTsVector/Matches translate to @@ / to_tsquery).
+/// (websearch_to_tsquery / @@ / ts_rank_cd). Results are relevance-ranked when a
+/// free-text query is present, date-ordered otherwise. See
+/// docs/discovery/05-search-redesign.md for the full multi-layer search design.
 /// </summary>
 public class SearchService : ISearchService
 {
@@ -58,16 +60,24 @@ public class SearchService : ISearchService
                 return new PagedResult<EmailSummaryDto> { Items = [], Page = r.Page, PageSize = r.PageSize, TotalCount = 0 };
         }
 
-        if (!string.IsNullOrWhiteSpace(r.Query))
-        {
-            // PostgreSQL full-text match against the generated tsvector.
-            var term = r.Query.Trim();
-            q = q.Where(e => e.SearchVector!.Matches(EF.Functions.PlainToTsQuery("english", term)));
-        }
+        // websearch_to_tsquery (vs. plainto_tsquery) understands quotes ("exact phrase"),
+        // OR, and -exclusions — the syntax users already expect from web search boxes.
+        var hasFreeTextQuery = !string.IsNullOrWhiteSpace(r.Query);
+        var term = r.Query?.Trim() ?? string.Empty;
+        if (hasFreeTextQuery)
+            q = q.Where(e => e.SearchVector!.Matches(EF.Functions.WebSearchToTsQuery("english", term)));
 
         var total = await q.CountAsync(ct);
-        var items = await q
-            .OrderByDescending(e => e.SentAtUtc)
+
+        // Relevance-ranked when there's a free-text query (ts_rank_cd via RankCoverDensity,
+        // recency as a tiebreaker); date-only otherwise — matches the existing browse
+        // behaviour when the user isn't searching for anything in particular.
+        var ranked = hasFreeTextQuery
+            ? q.OrderByDescending(e => e.SearchVector!.RankCoverDensity(EF.Functions.WebSearchToTsQuery("english", term)))
+               .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(