fix(memory): enforce usage store contracts - #7155

Summary

• allow records that exactly reach agent, global, provider cost, or provider token limits; reject only true excess
• tolerate normal floating-point rounding at exact decimal boundaries
• reject non-finite or negative usage costs and cost limits before persistence
• preserve typed memory errors when atomic usage checks cannot acquire a database connection
• bind daily-window modifiers instead of interpolating them into SQL
• fail agent daily rollups on corrupt agent IDs instead of silently hiding spend

Verification

• CARGO_INCREMENTAL=0 CARGO_TARGET_DIR=/Volumes/Lexar/worktrees/audit-usage-store-contract/target-7155 cargo test -p librefang-memory usage::tests:: --lib (20 passed after latest main merge)
• cargo fmt --check
• git diff --check
• earlier branch verification: cargo test -p librefang-memory
• earlier branch verification: cargo clippy -p librefang-memory --all-targets -- -D warnings
• earlier branch verification: cargo check --workspace --lib

Out of scope

• repairing already persisted corrupt agent IDs or invalid historical costs
• changing UTC accounting windows
• changing HTTP quota status mapping