quality(errors): implement custom exception hierarchy

[Snider]

Parent audit: #51

Finding

The framework uses generic exceptions (RuntimeException, InvalidArgumentException) instead of domain-specific exceptions, making error handling imprecise and catch blocks overly broad.

Fix

Implement a custom exception hierarchy with a base CoreException, then domain-specific exceptions (ModuleException, TenantException, ConfigException, etc.) that extend it.

Acceptance Criteria

• Base CoreException class created
• Domain-specific exceptions extend CoreException
• At minimum: ModuleException, ConfigException, ValidationException
• Each exception includes contextual data (module name, config key, etc.)
• Existing generic exception throws replaced with specific ones
• Exception handler maps custom exceptions to appropriate HTTP codes

[Snider]

Parent: #107