feat: list stored deals over REST and make export read them

[numarasSigmaSoftware]

Problem

Both booking paths persist deals under deal:<id>, but no route enumerates them. export_deals reads a deal_index key that nothing in the codebase writes, so GET /api/v1/deals/export always returns an empty list. Verified against a running instance on the CSV adapter: two deals booked through POST /api/v1/deals and POST /api/v1/deals/from-template, export still returned count: 0.

Change

• Add GET /api/v1/deals, operator key required, with an optional status filter typed as the shared DealStatus wire enum and applied to the wire-mapped status, so ?status=booked returns template-booked deals stored as confirmed. Returns DealListResponse: the existing DealBookingResponse envelope per deal, count, and skipped, the ids of stored records that fail to serialize (logged with their id; one bad row no longer fails the whole list).
• Scoping decision: the list is operator-only, the same posture as the rest of the admin surface. Buyers keep GET /api/v1/deals/{deal_id}. Export is unchanged, including its stored-status filter (confirmed / proposed / cancelled) that connectors already use.
• Back the list with deal_service.list_deals(), which uses the storage base's existing deal:* scan. export_deals now reads the same set and the dead deal_index lookup is gone.
• Regenerate docs/reference/endpoints.md; add the route to docs/api/overview.md; changelog entry under Unreleased.

Relies on #75 for the wire mapping of the template path's internal confirmed status. docs/api/openapi.json is not touched here: it is regenerated in #83, and this branch rebases onto that once merged and regenerates again so the list route lands in the document.

Verification

• tests/unit/test_deal_list_route.py: route registration, list, wire-status filter (including the 422 on an internal name), operator gate (401 anonymous), skipped rows, empty store, non-matching filter, a real scan on a temp SQLite backend with an idempotency key in the store, and export after booking through both POST routes. Each behavior test was seen failing before its change.
• tests/unit/test_route_shadowing.py: storage mock gained list_deals.
• Full unit suite on a fresh database: all pass. On a reused working directory one pre-existing test fails, test_self_asserted_advertiser_identity_is_floored, because the suite leaves ad_seller.db behind; that is Unit suite leaves ./ad_seller.db behind, so a second consecutive local run can fail #82 and is not touched by this PR.

Not changed

No pagination on the list yet, since no other list route in the API pages either. Endpoint counts in the hand-written docs are bumped in #83; one further bump follows once both merge.

Note for maintainers, not part of this change

tests/unit/test_deal_list_route.py copies the sys.modules stub for ad_seller.flows.execution_activation_flow that test_route_shadowing.py, test_deal_booking_endpoints.py, and test_trust_tier_verification.py already carry. Each copy exists to dodge the cancel-scope leak tracked in #60. #66 landed as part 2 of that issue, so the four stub blocks may now be removable. Left untouched here; a separate cleanup PR should delete all four once the suite is confirmed green without them.