- Add quota_warning to UploadSaveResponse
- Add warning to QuotaResponse
- Surface 80% quota warnings to clients after uploads
- Fix landing/build path for rust-embed

* phase6/cost-cache:
Phase 6: Cost calculation service with TTL cache

- Add api/src/cost/mod.rs with DashMap-based cost calculation cache
- Add GET /costs endpoint returning delta/base costs per game
- Add storage cost display to admin stats command
- Cache expires after 5 minutes to balance freshness vs performance
- Helps clients estimate upload costs before committing

- Add atomic reserve_quota() function with SELECT ... FOR UPDATE
- Replaces separate check_quota + increment_quota with single atomic operation
- Eliminates race condition between quota check and increment
- Add admin CLI commands: quota list, quota set, quota reset, quota show
- Track quota via gamer_quotas table
- Return HTTP 413 when quota exceeded on upload