feat(catalog): add bonus_items and metadata to FreeItem

[dinosoeren]

Adds two optional fields to the egress catalog FreeItem message so a free (claimable) catalog item can declare bonus rewards and forward webhook metadata:

• bonus_items — a repeated list of typed bonus rewards (type + id + amount) granted when the item is claimed. The only reward kind granted today is LOYALTY_POINTS; other/unspecified types are ignored by consumers.
• metadata — a map<string, string> of opaque key-value pairs forwarded to webhook events for the item, mirroring the existing product metadata field (not for display or business logic).

Both fields are additive and optional, so existing responses are unaffected. Regenerated the merged OpenAPI spec (docs/gen/swagger.v1.json).

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Note

Low Risk
Additive protobuf/OpenAPI contract changes only; no runtime logic in this diff, so backward compatibility is preserved for clients that omit the new fields.

Overview
Extends egress catalog FreeItem with two optional fields and regenerates swagger.v1.json.

bonus_items is a repeated FreeBonusItem (type, id, amount) describing extra rewards on claim (e.g. loyalty points). A new FreeBonusItemType enum is added; only LOYALTY_POINTS is granted today—other values are documented as ignored by consumers. amount is validated as a non-empty digit string for large integers.

metadata is a map<string, string> for opaque key-value data forwarded on related webhook events, aligned with purchasable product metadata (not for UI or business logic).

Both fields are additive; existing FreeItem payloads stay valid without them.

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