feat(search): weight subject above body in FTS vector (#14)

src/InboxIntel.Infrastructure/Persistence/AppDbContext.cs

@@ -59,15 +59,17 @@ public class AppDbContext : DbContext, IAppDbContext
         modelBuilder.Entity<UnsubscribeItem>().HasQueryFilter(e => CurrentUserId == Guid.Empty || e.UserId == CurrentUserId);
 
         // PostgreSQL full-text search: generated tsvector over subject + body with a
-        // GIN index, maintained by the DB and read-only in code. The tsvector type is
-        // Npgsql-only, so map it only for relational providers and ignore it otherwise
-        // (e.g. the InMemory provider used by tests). Production behaviour is unchanged.
+        // GIN index, maintained by the DB and read-only in code. Subject is weighted 'A'
+        // and body 'B' so ts_rank_cd ranks a subject match above a body-only mention. The
+        // tsvector type is Npgsql-only, so map it only for relational providers and ignore
+        // it otherwise (e.g. the InMemory provider used by tests).
         if (Database.IsRelational())
         {
             modelBuilder.Entity<Email>().Property(e => e.SearchVector)
                 .HasColumnType("tsvector")
                 .HasComputedColumnSql(
-                    "to_tsvector('english', coalesce(\"Subject\",'') || ' ' || coalesce(\"BodyText\",''))",
+                    "setweight(to_tsvector('english', coalesce(\"Subject\",'')), 'A') || " +
+                    "setweight(to_tsvector('english', coalesce(\"BodyText\",'')), 'B')",
                     stored: true);
             modelBuilder.Entity<Email>().HasIndex(e => e.SearchVector).HasMethod("GIN");
         }