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/migrations/009_module_licensing.sql

@@ -0,0 +1,55 @@
+-- Module Licensing System
+-- Phase 4: Module marketplace and license-module binding
+
+-- Module registry (available modules in marketplace)
+CREATE TABLE modules (
+    id UUID PRIMARY KEY DEFAULT uuid_generate_v4(),
+    slug VARCHAR(100) UNIQUE NOT NULL,
+    name VARCHAR(200) NOT NULL,
+    description TEXT,
+    category VARCHAR(50), -- 'loot', 'events', 'economy', 'kits', etc.
+    price_usd DECIMAL(10,2) NOT NULL,
+    preview_image_url TEXT,
+    screenshots JSONB,
+    features JSONB,
+    version VARCHAR(20) NOT NULL,
+    plugin_file_url TEXT,  -- Download URL for plugin file
+    created_at TIMESTAMPTZ DEFAULT NOW()
+);
+
+-- Module purchases (which licenses own which modules)
+CREATE TABLE module_purchases (
+    id UUID PRIMARY KEY DEFAULT uuid_generate_v4(),
+    license_id UUID NOT NULL REFERENCES licenses(id) ON DELETE CASCADE,
+    module_id UUID NOT NULL REFERENCES modules(id) ON DELETE CASCADE,
+    purchased_at TIMESTAMPTZ DEFAULT NOW(),
+    transaction_id VARCHAR(255),  -- PayPal transaction ID
+    amount_paid DECIMAL(10,2),
+    UNIQUE(license_id, module_id)
+);
+
+-- Module installations (deployment status on servers)
+CREATE TABLE module_installations (
+    id UUID PRIMARY KEY DEFAULT uuid_generate_v4(),
+    license_id UUID NOT NULL REFERENCES licenses(id) ON DELETE CASCADE,
+    module_id UUID NOT NULL REFERENCES modules(id) ON DELETE CASCADE,
+    status VARCHAR(50) DEFAULT 'pending', -- 'pending', 'installing', 'installed', 'failed'
+    installed_at TIMESTAMPTZ,
+    error_message TEXT,
+    UNIQUE(license_id, module_id)
+);
+
+CREATE INDEX idx_module_purchases_license ON module_purchases(license_id);
+CREATE INDEX idx_module_installations_license ON module_installations(license_id);
+
+-- Seed data: Loot Manager module
+INSERT INTO modules (slug, name, description, category, price_usd, version, features)
+VALUES (
+    'loot-manager',
+    'Loot Manager',
+    'Visual loot table editor with drag-and-drop items, spawn rates, and one-click profile switching.',
+    'loot',
+    9.99,
+    '1.0.0',
+    '["Visual loot table editor", "Container browser (crates, barrels, NPCs)", "Loot profiles (2x, 10x, etc.)", "One-click profile switching", "Import/export profiles"]'::JSONB
+);