Feat/max plans per merchant 51

[manueldezman]

Closes #51

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Summary

Implements Issue #51 by enforcing a maximum number of subscription plans a merchant can create, with a global default cap and an admin-updatable on-chain override. Existing plans remain unaffected; once a merchant reaches the cap, further plan creation fails with a clear error.

Changes

• Enforce per-merchant plan cap in create_plan (default MAX_PLANS_PER_MERCHANT = 100)
• Add global config storage key StorageKey::MaxPlansPerMerchant
• Add admin APIs:
  • set_max_plans_per_merchant(new_limit: u32)
  • get_max_plans_per_merchant() -> u32
• Add proxy passthrough wrappers for set/get
• Add integration tests verifying:
  • 3rd plan fails when limit is 2
  • lowering the limit does not affect existing plans but blocks new creates

Pull Request Checklist

Quality Gates (All must pass before merge)

• Lint: Code passes ESLint and Prettier checks
• Type Check: TypeScript compilation succeeds
• Tests: All tests pass (cd contracts && cargo test)
• Build: Project builds successfully
• Rust Format: Smart contract formatting is correct
• Rust Clippy: Smart contract linting passes
• Rust Tests: All smart contract tests pass (cd contracts && cargo test)
• Rust Build: Smart contracts compile successfully (covered by cargo test)

Additional Requirements

• New code has appropriate TypeScript types
• No hardcoded secrets or credentials
• New features have corresponding tests
• Documentation updated if needed

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[Smartdevs17]

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