Implement plugin system for alternative AI providers and templates

[saviobatista]

Business Value
Enabling plugin support gives users flexibility to bring their own AI provider or résumé styling logic. This makes the platform more extensible and community-driven.

Description
Design and implement a plugin system that allows:

• Swapping AI provider (e.g. Claude, local LLM)
• Registering custom LaTeX templates
• Replacing parser or generator logic

Technical Details

• Use simple factory pattern or plugin registry
• Support loading plugins from /lib/plugins
• Define plugin interface types for AI, template, parser

Acceptance Criteria

• Plugins can be defined and loaded dynamically
• At least 1 alternate AI provider plugin supported
• Custom template plugin loads and renders correctly