Update existing tenant-specific validation rules.

**Use Case:** Adjust rule logic, severity, or requirements based on operational feedback.

**Authentication:** API Key (X-API-Key header)
**Required Scope:** `rules:write`

**Request Structure:**
- scope: "tenant"
- rules: List of rules to modify (must include rule_id)

**Modifiable Fields:**
- requirement: Update description
- severity: Change severity level (ERROR/WARNING/INFO)
- applicability: Update logic
- category: Update category

**Example Request:**
```json
{
    "scope": "tenant",
    "rules": [
        {
            "rule_id": "rule-uuid-123",
            "requirement": "Updated requirement text",
            "severity": "WARNING"
        }
    ]
}
```

**Example Response:**
```json
{
    "success": True
}
```