feat: add inventory signals to availability schema

[hemanth]

Closes #345

Summary

Extends the availability schema with optional inventory signals: stock quantity and restock ETA.

Changes

• source/schemas/shopping/types/availability.json:
  • Added low_stock to well-known status values
  • Added optional quantity field (integer, min 0) for stock count
  • Added optional restock_at field (RFC 3339 datetime) for restock estimates

Design decisions

• All new fields are optional — businesses that don't track inventory just omit them
• The existing available boolean remains the simple fallback
• low_stock added as a well-known status value (thresholds are a business decision)
• Dropped location-level inventory from the issue proposal to keep this minimal; location support can be added via the existing retail_location type in a follow-up
• Especially useful for voice/agent scenarios where stock signals guide buyer decisions