Simplify messages_count trigger install and harden ensure!

Require all three SQLite triggers before treating the counter as installed,
drop the schema.rb / dump-rewrite install paths in favor of rake ensure after
schema load, isolate destructive trigger tests, and cover foreign room moves
plus fixture baseline counts.

Co-authored-by: Thomas Klemm <github@tklemm.eu>

test/controllers/messages/by_bots_controller_test.rb

@@ -103,7 +103,6 @@ class Messages::ByBotsControllerTest < ActionDispatch::IntegrationTest
 
     json = JSON.parse(response.body)
     assert_equal Message::PAGE_SIZE, json.size
-    assert_equal @room.reload.messages_count.to_s, response.headers["X-Total-Count"]
     assert_equal "41", response.headers["X-Total-Count"]
     assert_not_includes json.map { it["id"] }, messages(:fourth).id