Add Rate Limiting to Smart Contracts

## Overview
Add rate limiting mechanisms to smart contracts to prevent spam and DoS attacks.

## Background
Currently, contracts don't implement any rate limiting. Malicious actors could spam contracts with transactions, potentially causing DoS or excessive storage growth.

## Proposed Solution
Implement rate limiting using block numbers:

```rust
last_action: Mapping,
min_blocks_between_actions: BlockNumber,

fn check_rate_limit(&self, account: AccountId) -> Result<()> {
if let Some(last_block) = self.last_action.get(account) {
let current_block = self.env().block_number();
if current_block - last_block < self.min_blocks_between_actions {
return Err(Error::RateLimitExceeded);
}
}
Ok(())
}
```

## Tasks
- [ ] Design rate limiting strategy (per-contract or global)
- [ ] Implement in access_registry
- [ ] Implement in attribute_store
- [ ] Implement in policy_engine
- [ ] Implement in payment_integration
- [ ] Add tests for rate limit scenarios
- [ ] Document rate limit parameters

## Related
- Code review finding: M3

## Priority
MAJOR - Security enhancement

## Estimated Effort
1-2 days

Start by locating the access_registry, attribute_store, policy_engine, and payment_integration smart contracts and review their existing transaction entry points. Resolve whether rate limiting is per-contract or global, then inspect the contract test structure before defining scenarios. Done means all four contracts enforce the chosen block-based limits, rate-limit scenarios pass, and the parameters are documented.

Written by the indexing model from the issue text.