[CO-10] Implement emergency pause mechanism across all four contracts

[Leothosine]

Problem

None of the four contracts (loyalty_token, payment, order, restaurant_registry) have a pause or circuit-breaker mechanism. Soroban contracts cannot be patched after deployment - only migrated. If a critical vulnerability is discovered post-launch, there is no way to halt operations while a fix is deployed. Exploitation continues unimpeded until a full contract migration is complete.

Proposed Solution

Add a pause mechanism to all four contracts following the same pattern:

1. Add Paused variant to each contract's DataKey enum
2. Add two admin-only functions to each contract:

pub fn pause_contract(env: Env, caller: Address) {
    caller.require_auth();
    Self::assert_admin_or_panic(&env, &caller);
    env.storage().instance().set(&DataKey::Paused, &true);
    env.events().publish((symbol_short!("ctrl"), symbol_short!("pause")), env.ledger().timestamp());
}

3. Add a guard at the top of every public state-mutating function - admin can operate when paused, all other callers are rejected with "contract is paused"

Acceptance Criteria

• All four contracts have pause_contract() and unpause_contract()
• Non-admin callers on a paused contract receive a panic with "contract is paused"
• Admin can call state-mutating functions even when the contract is paused
• unpause_contract() restores normal operation for all callers
• Each contract emits a timestamped pause/unpause event
• Unit test per contract: pause ? user call panics ? admin call succeeds ? unpause ? user call succeeds

[Menjay7]

Hi! I came across this issue and would love to work on it. I have experience with similar tasks and I'm confident I can investigate, implement a clean solution, and thoroughly test it before submitting a PR. If the issue is still open, I would appreciate it if you could assign it to me. Thank you!

[OZILSOLAR]

Hi team!!!!
I would like to work on this issue!!!!!

[jaynomyaro]

Hi! I’m a full-stack developer with a strong blockchain background. I’d love to handle this issue and can deliver a clean, reliable solution. Kindly assign it to me.

[Senorespecial]

I would really like to work on this issue, please assign.

[prismn]

I have experience with Rust and can implement the emergency pause mechanism, Can i be assigned ?

[grantfox-oss]

🦊 GrantFox — @prismn has been assigned to this issue as part of the Official Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #288)
2. Your PR will be reviewed by the tosirano maintainers

Good luck! Track your progress on GrantFox.

[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign campaign!

@prismn's PR #297 was approved and merged by @Leothosine.

🏆 @prismn: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (611 total points). Track your full progress on GrantFox.

👏 Great work, @prismn! Keep contributing to tosirano.