feat: add Life Weeks domain logic, models, and scheduler

[glebis]

Summary

• Adds calculate_life_week() pure function for week-from-birth-date calculation
• Adds LifeWeekConfig and LifeWeekEntry SQLAlchemy models
• Adds should_notify() scheduler decision logic and format_life_week() display formatting
• Foundation for the Life Weeks feature trilogy (feat: Life Weeks scheduler and database schema #99 → feat: Life Weeks reply context and vault routing #100 → feat: Life Weeks settings UI and plugin settings infrastructure #101)

Test plan

• 20 tests covering week calculation, model instantiation, formatting, and notification logic
• Verify models can be created in SQLite after migration

Closes #99

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