[P4][SHOP-04] Auctions and maximum commitments

Programme: #1
Epic: #27

## Objective
Support marketplace bidding safely using explicit maximum commitments and a global reservation ledger so simultaneous auctions cannot exceed the user's total budget.

## Scope
- Auction entity: marketplace/account/item/variant, current price, currency, fees, end time, bid increment and observed time.
- Distinguish current price from user's maximum bid/commitment.
- Bid preparation/refresh/approval/commit through TX-01.
- Reserve worst-case configured commitments across all outstanding auctions through TX-02.
- Handle auction end, outbid, won/lost, cancellation limitations and stale price/time data.
- Provider production-access requirements and test-account/sandbox strategy.
- Notifications for outbid/end/win/unknown outcomes.

## Safety rules
- A submitted bid may be irrevocable; local cancellation cannot claim to withdraw it.
- Global commitment includes relevant known fees/currency conversions according to policy.
- Timeout after bid submission enters reconciliation, not automatic rebid.

## Acceptance criteria
- [ ] Fixture tests preserve global budget across simultaneous auctions.
- [ ] Maximum bid is displayed/authorized separately from current price.
- [ ] End time/currency/known fees and source freshness are included before approval.
- [ ] Ambiguous bid submission never causes an automatic duplicate bid.
- [ ] Win/loss/outbid state is verified against provider truth.
- [ ] Live support is enabled only where CONN-04 records required production approval.

## Dependencies
- SHOP-01
- TX-02
- CONN-04

**First phase:** P4
**Maturity target:** P7
**Owner:** connectors-domains

Start by reading the dependencies SHOP-01, TX-02, and CONN-04, then review the fixture tests for simultaneous auctions. The acceptance criteria define done: preserve the global budget, separate maximum bids from current prices, verify provider outcomes, avoid duplicate bids after ambiguity, and gate live support on CONN-04 approval.

Written by the indexing model from the issue text.