libstore: add SQLite::Settings struct for explicit configuration

Progress on #5638

Replace the SQLite constructor's mode parameter with a Settings struct
that includes both the open mode and useWAL flag. This makes the
dependency on useSQLiteWAL explicit at call sites rather than having
it read from the global settings inside the constructor.

All call sites now explicitly pass settings.useSQLiteWAL, preparing
for downstream work where stores can pass their own settings instead
of relying on the global.

src/libexpr/eval-cache.cc

@@ -75,7 +75,7 @@ struct AttrDb
 
         auto dbPath = cacheDir / (fingerprint.to_string(HashFormat::Base16, false) + ".sqlite");
 
-        state->db = SQLite(dbPath);
+        state->db = SQLite(dbPath, {.useWAL = settings.useSQLiteWAL});
         state->db.isCache();
         state->db.exec(schema);