add SQLite concurrency tests and bench task

BenchRepo for isolated concurrency testing against temp DB files.
Correctness tests prove WAL concurrent reads, IMMEDIATE transaction
mode vs DEFERRED upgrade failures, and PRAGMA application. Benchmark
tests (tagged :benchmark, excluded by default) measure throughput.

mix bench.sqlite runs HTTP load scenarios against the full Phoenix
stack with --prod, --scale, --pool-size, and --busy-timeout options.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

test/test_helper.exs

@@ -1,2 +1,2 @@
-ExUnit.start()
+ExUnit.start(exclude: [:benchmark])
 Ecto.Adapters.SQL.Sandbox.mode(Berrypod.Repo, :manual)