tune SQLite PRAGMAs for production

Add journal_size_limit (64MB), default_transaction_mode: :immediate,
and mmap_size (128MB) across dev, test, and prod configs. Benchmarks
showed IMMEDIATE mode eliminates transaction upgrade BUSY errors and
mmap improves read throughput.

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

config/dev.exs

@@ -6,6 +6,9 @@ config :berrypod, Berrypod.Repo,
   pool_size: 5,
   journal_mode: :wal,
   busy_timeout: 5000,
+  journal_size_limit: 67_108_864,
+  default_transaction_mode: :immediate,
+  custom_pragmas: [mmap_size: 134_217_728],
   stacktrace: true,
   show_sensitive_data_on_connection_error: true