Add per-table concurrent timeout overrides

[mathyoudawson]

Summary

• Adds an optional table_overrides config hash to register_database, letting apps override concurrent_lock_timeout/concurrent_statement_timeout for specific tables (e.g. a huge payments table needing a longer statement timeout than the database-wide default).
• Falls back to the database-level value for any table (or key) not listed. Per-migration set_lock_timeout/set_statement_timeout still takes precedence over both.
• Fully backward compatible: table_overrides defaults to {}, no behaviour changes for existing configs.

Nandi.configure do |config|
  config.register_database(:primary,
    concurrent_statement_timeout: 600_000,   # 10 min database-wide default
    table_overrides: {
      payments: { concurrent_statement_timeout: 1_800_000 }, # 30 min for payments
    })
end

Bumps version to 3.2.0.

Test plan

• bundle exec rspec — 447 examples, 0 failures
• bundle exec rubocop — 0 offenses
• New specs added for MultiDatabase::Database, Config, and Migration covering: override present/absent, partial-key overrides, string/symbol table names, per-migration override precedence, and multi-database database_name + table resolution together

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