[34] Add a per-course reward configuration replacing the hardcoded `BASE_REWARD_AMOUNT`

[merlik787-droi]

Problem Statement

course-registry::complete_module hardcodes let base_reward: i128 = 10_0000000; (line 329). Every course pays the same reward on completion. There's no field on Course to specify its reward amount, despite the constant BASE_REWARD_AMOUNT: i128 = 10_0000000 existing at the crate level.

Why It Matters

• Every course paying the same amount kills curriculum differentiation.
• A premium tier of courses cannot pay more without a contract upgrade.

Expected Outcome

• Add reward_amount: i128 to the Course struct.
• create_course accepts a reward amount and validates it (> 0, < MAX_*).
• complete_module reads course.reward_amount instead of constant.
• New admin-only set_course_reward(env, admin, id, new_reward) (with safeguards).
• Migration: existing courses get a default zero reward; admin must call set_course_reward to enable.

Acceptance Criteria

• Different courses can have different reward amounts.
• Migration is non-destructive.
• Tests for the new fields and setter.

Implementation Notes

• Watch the Soroban storage layout change — bump a ContractVersion instance key as a marker.
• Tie storage migration to Issue [35] Add storage versioning and migrations to all contracts #35 (storage versioning).

Files / Modules Affected

• contracts/course-registry/src/types.rs
• contracts/course-registry/src/lib.rs
• contracts/course-registry/src/test.rs
• contracts/course-registry/Cargo.toml (version bump).

Dependencies

Issue #35.

Difficulty

Medium.

Estimated Effort

1–2 days.

Suggested Labels

feature, breaking-change, P2, course-registry

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[grantfox-oss]

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