add support for reviews

[jackhogan]

Implements a scheduler ported from Anki's Rust codebase:

• FSRS-6 memory model (fsrs.py) with parameter upgrade/clipping and SM-2 approximation
• Complete state machine (scheduler.py) covering new/learning/review/relearning states
• Answer bookkeeping: revlog entries, deck stats, leech handling, sibling burying
• Simplified next_card queue with daily limits and due-order gathering
• Collection timing with timezone-aware day boundary calculations
• Full oracle parity test suite validates scheduling against desktop Anki