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>

config/description.exs

@@ -3565,6 +3565,21 @@ config :pleroma, :config_description, [
       }
     ]
   },
+  %{
+    group: :pleroma,
+    key: Pleroma.Search.DatabaseSearch,
+    type: :group,
+    description: "Built-in database search settings.",
+    children: [
+      %{
+        key: :gin_fuzzy_search_limit,
+        type: :integer,
+        description:
+          "Limits how many matches PostgreSQL considers in GIN full-text search queries (see `gin_fuzzy_search_limit` in the PostgreSQL docs). Only set for search queries. Keeps search fast on large databases, but results become incomplete and change between requests. Set to `nil` for exact results, which is fine on small instances. Default: 500.",
+        suggestions: [500, nil]
+      }
+    ]
+  },
   %{
     group: :pleroma,
     key: Pleroma.Search.Meilisearch,