Checkpoint SQLite WAL off the request thread safely

Disable wal_autocheckpoint and run PASSIVE checkpoints from one elected
writer process. Contenders start in console/rake (initializer), Puma
workers (including WEB_CONCURRENCY=auto), and Resque children, with
failover, capped error backoff, and tests for lock takeover.

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

config/initializers/sqlite_wal_checkpoint.rb

@@ -0,0 +1,10 @@
+# Start a checkpoint contender in console, runner, rake and other non-Puma writers.
+# Puma skips this path: config/puma.rb and config/puma_dev.rb start after the
+# correct process is chosen (worker boot vs single-process), so the master that
+# only forks workers never holds the lock alone. Resque starts after_prefork.
+Rails.application.config.after_initialize do
+  next if Rails.env.test?
+  next if defined?(Puma::CLI)
+
+  SqliteWalCheckpoint.start
+end