Add type hints to core modules

[aquarion]

Add type annotations to the core lifestream modules for better IDE support and code quality:

Modules to annotate

• imports/lifestream/db.py - EntryStore, get_connection, get_cursor
• imports/lifestream/utils.py - Utility functions
• imports/lifestream/cache.py - Cache functions

Benefits

• Better IDE autocompletion
• Catches type errors early
• Improved documentation

Notes

• Consider using from __future__ import annotations for forward references
• May need to add py.typed marker file