fix(search): set gin_fuzzy_search_limit only for search queries

The limit was a connection parameter, so it applied to every GIN index
scan, not only full-text search: recipient lookups and Oban job queries
could return incomplete, random results. Set it with SET LOCAL around
the search query only, configurable under Pleroma.Search.DatabaseSearch.

The default stays 500 for search: measured on a 74M-object database,
higher limits or none make searches for common words exceed the query
timeout. Remote fetches for URL queries run outside the transaction.

Based on Akkoma 811f0a468.

Co-authored-by: Oneric <oneric@oneric.stub>

test/pleroma/search/database_search_test.exs

@@ -44,4 +44,31 @@ defmodule Pleroma.Search.DatabaseSearchTest do
 
     assert result.id == other_post.id
   end
+
+  describe "gin_fuzzy_search_limit" do
+    defp fuzzy_limit do
+      %{rows: [[value]]} = Pleroma.Repo.query!("SHOW gin_fuzzy_search_limit")
+      value
+    end
+
+    test "is not set on normal database connections" do
+      assert fuzzy_limit() == "0"
+    end
+
+    test "is only set for the search query when configured" do
+      clear_config([Search, :gin_fuzzy_search_limit], nil)
+      assert Search.with_fuzzy_limit(&fuzzy_limit/0) == "0"
+
+      clear_config([Search, :gin_fuzzy_search_limit], 10_000)
+      assert Search.with_fuzzy_limit(&fuzzy_limit/0) == "10000"
+    end
+
+    test "search still works with a limit" do
+      clear_config([Search, :gin_fuzzy_search_limit], 10_000)
+      {:ok, post} = CommonAPI.post(insert(:user), %{status: "it's wednesday my dudes"})
+
+      assert [%{id: id}] = Search.search(nil, "wednesday")
+      assert id == post.id
+    end
+  end
 end