Critical fixes:
- Add admin API key authentication for admin endpoints
- Add race condition protection via unique partial index for starter decks
- Make starter deck selection atomic with combined method
Moderate fixes:
- Fix DI pattern violation in validate_deck_endpoint
- Add card ID format validation (regex pattern)
- Add card quantity validation (1-99 range)
- Fix exception chaining with from None (B904)
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>