feat: database transaction support with ScalaSql

[vim89]

Add database transaction decorator with direct ScalaSql dependency.

Features:

• @cask.database.transactional decorator for automatic transaction management
• Direct compile-time dependency on ScalaSql (no ClassTag reflection)
• Requires Scala 3.7+ with named tuples support
• Auto-commits on success, rolls back on exceptions or HTTP 4xx/5xx responses

Changes:

• Add scalasql-namedtuples dependency for Scala 3.7+
• Hardcode ScalaSql DbClient type (no generic type parameter)
• Restore scalasql.DbApi.Txn import in examples for cleaner code
• Add stub implementations for Scala 2.12/2.13 compatibility
• Update todoDb examples to demonstrate transaction usage

Review Feedback Addressed:

• Removed ClassTag and reflection-based approach
• Hardcoded ScalaSql dependency as suggested
• Restored DbApi.Txn import to reduce verbosity

Addresses database aspect of #181 and #183

[vim89]

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