feat: Implement Phase 4 module licensing backend

Build complete module activation and license-module binding system with
marketplace catalog, purchase tracking, and installation status monitoring.

Database schema (migration 009):
- modules table — Registry with pricing, features, plugin URLs
- module_purchases — License-module ownership with transaction logging
- module_installations — Deployment status tracking
- Seed data: Loot Manager module ($9.99)

Backend implementation:
- Domain models with rust_decimal pricing support
- 11 data access functions (catalog, ownership, purchases, installation)
- 5 REST endpoints with JWT auth and license scoping
- Multi-tenant enforcement via license_id from claims

Purchase flow stub:
- Immediate purchase recording without payment gateway
- PayPal integration deferred to XO's direct implementation
- Transaction ID and amount fields ready for real gateway

Module installation:
- Integration with ModuleInstaller service
- NATS-based deployment to companion agent
- Real-time status tracking via polling endpoint

All queries compile-time verified. Zero cross-tenant exposure.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

backend/src/main.rs

@@ -131,6 +131,7 @@ async fn main() -> anyhow::Result<()> {
         .nest("/api/analytics", api::analytics::router())
         .nest("/api/plugin", api::plugin::router())
         .nest("/api/settings", api::settings::router())
+        .nest("/api/modules", api::modules::router())
         .layer(cors)
         .layer(TraceLayer::new_for_http())
         .with_state(state);