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>

CHANGELOG.md

@@ -67,6 +67,68 @@ All notable changes to this project will be documented in this file.
 
 **Status:** Skeleton complete. Hooks functional. Profile switching works via chat command. Dashboard UI integration and deployment automation pending future iteration.
 
+### Added (Phase 4 — Module Auto-Installation Pipeline)
+
+**Backend Service:**
+- `backend/src/services/module_installer.rs` — Automated module deployment orchestrator:
+  - `ModuleInstaller::install_module(license_id, module_id)` — Main entry point
+  - Purchase verification against `module_purchases` table
+  - Module metadata fetch (plugin_file_url, slug)
+  - Server connection detection (AMP, Pterodactyl, bare metal)
+  - Multi-adapter dispatch with automatic failover
+  - Installation status tracking (pending → installing → installed/failed)
+  - Background task spawning for async installation
+- Panel adapter integration:
+  - `install_via_amp()` — Downloads plugin, uploads to `oxide/plugins/`, executes `oxide.reload *`
+  - `install_via_pterodactyl()` — Same flow using Pterodactyl client API
+  - `install_via_companion()` — Publishes NATS command to bare metal agent
+- HTTP client integration: `reqwest` for plugin file download from CDN
+- Encryption support: Decrypts panel API keys using `services::encryption::decrypt()`
+- Error handling: Comprehensive context wrapping with installation failure logging
+
+**NATS Integration:**
+- New subject pattern: `corrosion.{license_id}.cmd.module.install`
+- Request/reply timeout: 60 seconds for companion agent response
+- Expected payload:
+  ```json
+  {
+    "module_id": "loot-manager",
+    "download_url": "https://cdn.corrosionmgmt.com/modules/LootManager.cs",
+    "filename": "LootManager.cs",
+    "target_path": "oxide/plugins/"
+  }
+  ```
+- Expected response:
+  ```json
+  {
+    "module_id": "loot-manager",
+    "success": true|false,
+    "error": "optional error message"
+  }
+  ```
+- Subject pattern already covered by existing `corrosion.*.cmd.>` wildcard in STREAM_AGENT_COMMANDS
+
+**API Updates:**
+- `backend/src/api/modules.rs`:
+  - Updated `POST /api/modules/install` — Replaced stub with real ModuleInstaller invocation
+  - Spawn background task for async installation
+  - Return immediately with "installing" status
+  - `GET /api/modules/:module_id/installation-status` — Already existed, now returns real data from `module_installations` table
+- ModuleInstaller instantiation with encryption key from AppConfig
+
+**Documentation:**
+- `docs/COMPANION_AGENT_MODULE_INSTALL.md` — Companion agent NATS contract specification:
+  - Subject patterns and payload schemas
+  - Expected agent behavior (download, install, reload, respond)
+  - Error handling requirements
+  - Example pseudocode implementation (Go)
+  - Testing procedures and failure scenarios
+
+**Dependencies:**
+- `Cargo.toml`: Added `rust_decimal` feature to `sqlx` for DECIMAL field support
+
+**Status:** Backend pipeline fully operational. Modules install automatically to AMP/Pterodactyl servers. Companion agent NATS contract documented. Companion agent implementation (Go) pending future iteration.
+
 ### Added (Phase 4 — Module Store Frontend)
 
 **Frontend:**