docs: add observability, creating modules guide, and unified migration plan

- Add observability framework documentation (health checks, metrics, Sentry)
- Add developer guide for creating modules
- Add comprehensive module migration plan with Celery task integration
- Update architecture overview with module system and observability sections
- Update module-system.md with links to new docs

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

docs/architecture/overview.md

@@ -76,7 +76,42 @@ Custom middleware handles:
 
 **See:** [Authentication & RBAC](auth-rbac.md) for details
 
-### 4. Request Flow
+### 4. Module System
+
+The platform uses a modular architecture with three-tier classification:
+
+```
+┌─────────────────────────────────────────────────────────────┐
+│                    FRAMEWORK LAYER                          │
+│  (Infrastructure - always available, not modules)           │
+│  Config │ Database │ Auth │ Permissions │ Observability     │
+└─────────────────────────────────────────────────────────────┘
+                            │
+                            ▼
+┌─────────────────────────────────────────────────────────────┐
+│                     MODULE LAYER                            │
+│                                                             │
+│  CORE (Always Enabled)     OPTIONAL (Per-Platform)         │
+│  ├── core                  ├── payments                    │
+│  ├── tenancy               ├── billing                     │
+│  ├── cms                   ├── inventory                   │
+│  └── customers             ├── orders                      │
+│                            ├── marketplace                 │
+│  INTERNAL (Admin-Only)     ├── analytics                   │
+│  ├── dev-tools             └── messaging                   │
+│  └── monitoring                                            │
+└─────────────────────────────────────────────────────────────┘
+```
+
+**Module Features:**
+- Enable/disable modules per platform
+- Module dependencies (billing requires payments)
+- Health checks and lifecycle hooks
+- Self-contained modules with own services, models, migrations
+
+**See:** [Module System](module-system.md) for complete documentation
+
+### 5. Request Flow
 
 ```mermaid
 graph TB
@@ -252,7 +287,15 @@ project/
 │   ├── api/               # API routes
 │   ├── routes/            # Page routes (HTML)
 │   ├── services/          # Business logic
-│   ├── core/              # Core functionality
+│   ├── core/              # Core functionality (config, db, observability)
+│   ├── modules/           # Module definitions and self-contained modules
+│   │   ├── base.py       # ModuleDefinition class
+│   │   ├── registry.py   # Module registry (CORE, OPTIONAL, INTERNAL)
+│   │   ├── service.py    # Module enablement service
+│   │   ├── events.py     # Module event bus
+│   │   ├── cms/          # Self-contained CMS module
+│   │   ├── payments/     # Self-contained payments module
+│   │   └── ...           # Other modules
 │   └── exceptions/        # Custom exceptions
 │
 ├── middleware/            # Custom middleware
@@ -288,6 +331,27 @@ project/
 
 ## Monitoring & Observability
 
+The platform includes a comprehensive observability framework:
+
+### Health Checks
+
+- Aggregated health endpoint (`/health`)
+- Kubernetes probes (`/health/live`, `/health/ready`)
+- Module health check integration
+- External tool links (`/health/tools`)
+
+### Metrics
+
+- Prometheus metrics endpoint (`/metrics`)
+- Request latency histograms
+- Counter and gauge support
+
+### Error Tracking
+
+- Sentry integration for production
+- Exception capture with context
+- Environment-aware configuration
+
 ### Logging
 
 - Structured logging with Python logging module
@@ -295,12 +359,7 @@ project/
 - Error tracking with stack traces
 - Audit logging for admin actions
 
-### Performance Monitoring
-
-- Request timing headers (`X-Process-Time`)
-- Database query monitoring (SQLAlchemy echo)
-- Slow query identification
-- Memory usage tracking
+**See:** [Observability](observability.md) for complete documentation
 
 ## Deployment Architecture
 
@@ -332,6 +391,9 @@ Internet ───────────│ Load Balancer│
 ## Related Documentation
 
 - [Multi-Tenant System](multi-tenant.md) - Detailed multi-tenancy implementation
+- [Module System](module-system.md) - Module architecture and classification
+- [Menu Management](menu-management.md) - Sidebar and menu configuration
+- [Observability](observability.md) - Health checks, metrics, error tracking
 - [Middleware Stack](middleware.md) - Complete middleware documentation
 - [Authentication & RBAC](auth-rbac.md) - Security and access control
 - [Request Flow](request-flow.md) - Detailed request processing