Add Pausability and Emergency Controls to Contracts

## Overview
Implement emergency pause functionality for all contracts to handle critical situations.

## Background
Contracts currently have no emergency stop mechanism. Adding pausability allows quick response to security issues.

## Proposed Implementation
```rust
pub struct AccessRegistry {
// ... existing fields
paused: bool,
}

#[ink(message)]
pub fn pause(&mut self) -> Result<()> {
if self.env().caller() != self.owner {
return Err(Error::NotOwner);
}
self.paused = true;
Ok(())
}
```

## Tasks
- [ ] Add pause/unpause functions to all contracts
- [ ] Add paused checks to critical functions
- [ ] Implement time-based auto-unpause
- [ ] Add emergency withdrawal functions if needed
- [ ] Write tests for paused state
- [ ] Document emergency procedures

## Related
- Code review suggestion 4.1 #1

## Priority
MEDIUM - Security enhancement

## Estimated Effort
2 days

The issue names no contract files, tests, or entry points; begin by locating all contracts and their critical functions. Review the existing ownership and state patterns before defining the pause, unpause, auto-unpause, withdrawal, testing, and documentation scope, then verify that every listed task has an agreed completion condition.

Written by the indexing model from the issue text.