Extract common key-value storage operations into internal/storage/kvtpl/
template directory. This consolidates duplicated logic from both leveldb
and inmemory implementations into reusable generic functions.
Key changes:
- Create kvtpl/ package with generic KV storage operations
- Consolidate iterator implementation using generics
- Update migration functions to use new template structure
- Refactor tests to work with new storage interface
This prepares the codebase for easier addition of new storage backends
by providing a consistent template layer.
