Overhaul temporal scoring with FSRS-inspired model (#11)

* refactor: overhaul temporal scoring with FSRS-inspired model

Replace ad-hoc temporal scoring with a principled model based on FSRS
(Free Spaced Repetition Scheduler) and Wozniak's Two Component Model.

Key changes:
- Remove access_count column entirely (redundant with stability)
- Write-only temporal bumps: reads are pure, writes strengthen knowledge
- Power-law forgetting curve: R = (1 + t/(S*d))^(-0.5)
- FSRS spacing effect: neglected knowledge grows more on write
- Multiplicative scoring with temporal floor (0.1)
- Simplify pruneStale to use retention * tierWeight directly
- Replace most/least accessed with most/least stable in health report

* fix: auto-migrate existing databases to drop access_count columns

Existing SQLite databases still have access_count columns from before
the temporal scoring overhaul. This adds an automatic migration in
initialize() that checks for and drops these columns on startup.

Bump version to 0.1.0