Fix buyer booking workflow and integrate GAM reporting

[Sirajmx]

GAM Delivery Reporting + Buyer Booking Workflow Fix

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1. GAM Delivery Reporting Integration

Adds read/reporting capabilities to the existing GAM SOAP client and exposes delivery data through two new seller-internal endpoints and the existing
deal performance endpoint.

gam_soap_client.py — Added 5 reporting methods: list_orders(), get_order_by_id(), list_line_items_for_order(), run_delivery_report(),
get_delivery_report(). Report jobs poll GAM until complete then parse CSV delivery rows. API version now validates against supported versions and
auto-upgrades retired ones.

main.py — Two new seller-internal endpoints:

• GET /gam/orders — lists live GAM orders with network/user context
• GET /gam/report — delivery report by order ID(s) (impressions, clicks, revenue)

GET /api/v1/deals/{deal_id}/performance — replaced placeholder with real GAM data when gam_order_id is present on the deal. Falls back to placeholder
when GAM is not configured or the deal has not been trafficked into GAM — the endpoint always responds.

settings.py — Default GAM_API_VERSION updated to v202505.

Docs — New guides/gam-reporting.md covering setup, both endpoints, deal-to-GAM linking via gam_order_id, fallback behaviour, and sequence diagram.
Stale v202411 references updated in configuration.md and inventory-sync.md.

Note: GAM credentials (GAM_ENABLED, GAM_NETWORK_CODE, GAM_JSON_KEY_PATH) are required for the real data path. Without them all endpoints degrade
gracefully — no breaking changes.

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2. Buyer Booking Workflow Fix

Resolves issues that caused the buyer agent's booking flow to fail when calling the seller.

main.py — Added POST /products/search and POST /products/avails endpoints which the buyer's channel research crews call during inventory discovery.
Without these, crew tool calls returned 404s and recommendations couldn't be generated.

product_setup_flow.py — Product IDs changed from random UUIDs (prod-a3f2c8b1) to stable sequential IDs (prod-001, prod-002, ...) so buyer crews can
reliably reference the same product across calls.

proposal_handling_flow.py — Added handle_proposal_async() using kickoff_async() so proposal evaluation no longer blocks the FastAPI event loop,
fixing timeout issues under concurrent buyer requests.