feat: Phase 4 module auto-installation + Phase 5 webstore backend

Phase 4 Contributions (Agent Golf):
- Module auto-installation service (module_installer.rs)
- NATS subject pattern for module installation commands
- Companion agent contract documentation
- API endpoint: POST /api/modules/install

Phase 5 XO Direct Touch:
- Webstore subscription API (PayPal recurring billing)
  * POST /api/webstore/subscription/create
  * GET /api/webstore/subscription
  * POST /api/webstore/subscription/cancel
  * POST /api/webstore/subscription/webhook
- Store configuration API (CRUD for store settings)
  * GET /api/webstore/config
  * PUT /api/webstore/config
- Store category/item management APIs (multi-tenant CRUD)
  * GET/POST/PUT/DELETE /api/webstore/categories
  * GET/POST/PUT/DELETE /api/webstore/items
- Public store API (customer-facing, subdomain-scoped)
  * GET /api/public-store/:subdomain
  * GET /api/public-store/:subdomain/items
  * POST /api/public-store/:subdomain/purchase
  * POST /api/public-store/:subdomain/webhook
- Transaction history API
  * GET /api/webstore/transactions
- Delivery system (NATS command execution on purchase)
- Migrations: payment_orders, webstore_subscriptions, store_config, store_items, store_transactions

Security:
- JWT auth + license_id scoping on admin endpoints
- Subdomain → license_id mapping on public endpoints
- Purchase limit enforcement
- Command injection prevention via placeholder replacement

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

backend/Cargo.lock

@@ -2897,6 +2897,7 @@ dependencies = [
  "memchr",
  "once_cell",
  "percent-encoding",
+ "rust_decimal",
  "rustls",
  "serde",
  "serde_json",
@@ -2981,6 +2982,7 @@ dependencies = [
  "percent-encoding",
  "rand 0.8.5",
  "rsa",
+ "rust_decimal",
  "serde",
  "sha1",
  "sha2",
@@ -3020,6 +3022,7 @@ dependencies = [
  "memchr",
  "once_cell",
  "rand 0.8.5",
+ "rust_decimal",
  "serde",
  "serde_json",
  "sha2",