feat: add marketplace bid increment anti-sniping

[dangelo352]

Summary

• add admin-configurable absolute minimum bid increment and anti-sniping settings
• enforce configured increments in place_bid with checked math and extend near-end bids up to a per-auction cap
• document the auction policy and add focused increment/extension/cap/default validation coverage

Fixes #470

Tests

• CARGO_TARGET_DIR=/tmp/commitlabs-bid-antisnipe-target cargo test -p commitment-marketplace auction -- --nocapture
• CARGO_TARGET_DIR=/tmp/commitlabs-bid-antisnipe-target cargo test -p commitment-marketplace place_bid -- --nocapture
• CARGO_TARGET_DIR=/tmp/commitlabs-bid-antisnipe-target cargo test -p commitment-marketplace update_auction_settings -- --nocapture
• CARGO_TARGET_DIR=/tmp/commitlabs-bid-antisnipe-target cargo test -p commitment-marketplace anti_sniping -- --nocapture