feat: implement module-driven context providers for dynamic page context

Introduces a module-driven context provider system that allows modules to
dynamically contribute template context variables without hardcoding imports.

Key changes:
- Add context_providers field to ModuleDefinition in app/modules/base.py
- Create unified get_context_for_frontend() that queries enabled modules only
- Add context providers to CMS module (PLATFORM, STOREFRONT)
- Add context providers to billing module (PLATFORM)
- Fix SQLAlchemy cross-module relationship resolution (Order, AdminMenuConfig,
  MarketplaceImportJob) by ensuring models are imported before referencing
- Document the entire system in docs/architecture/module-system.md

Benefits:
- Zero coupling: adding/removing modules requires no route handler changes
- Lazy loading: module code only imported when that module is enabled
- Per-platform customization: each platform loads only what it needs
- Graceful degradation: one failing module doesn't break entire page

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

app/modules/billing/definition.py

@@ -6,9 +6,57 @@ Defines the billing module including its features, menu items,
 route configurations, and scheduled tasks.
 """
 
+import logging
+from typing import Any
+
 from app.modules.base import MenuItemDefinition, MenuSectionDefinition, ModuleDefinition, PermissionDefinition, ScheduledTask
 from app.modules.enums import FrontendType
 
+logger = logging.getLogger(__name__)
+
+
+# =============================================================================
+# Context Providers
+# =============================================================================
+
+
+def _get_platform_context(request: Any, db: Any, platform: Any) -> dict[str, Any]:
+    """
+    Provide billing context for platform/marketing pages.
+
+    Returns pricing tier data for the marketing pricing page.
+    """
+    from app.core.config import settings
+    from app.modules.billing.models import TIER_LIMITS, TierCode
+
+    tiers = []
+    for tier_code, limits in TIER_LIMITS.items():
+        tiers.append({
+            "code": tier_code.value,
+            "name": limits["name"],
+            "price_monthly": limits["price_monthly_cents"] / 100,
+            "price_annual": (limits["price_annual_cents"] / 100)
+            if limits.get("price_annual_cents")
+            else None,
+            "orders_per_month": limits.get("orders_per_month"),
+            "products_limit": limits.get("products_limit"),
+            "team_members": limits.get("team_members"),
+            "features": limits.get("features", []),
+            "is_popular": tier_code == TierCode.PROFESSIONAL,
+            "is_enterprise": tier_code == TierCode.ENTERPRISE,
+        })
+
+    return {
+        "tiers": tiers,
+        "trial_days": settings.stripe_trial_days,
+        "stripe_publishable_key": settings.stripe_publishable_key,
+    }
+
+
+# =============================================================================
+# Router Lazy Imports
+# =============================================================================
+
 
 def _get_admin_router():
     """Lazy import of admin router to avoid circular imports."""
@@ -153,6 +201,10 @@ billing_module = ModuleDefinition(
         ],
     },
     is_core=False,  # Billing can be disabled (e.g., internal platforms)
+    # Context providers for dynamic page context
+    context_providers={
+        FrontendType.PLATFORM: _get_platform_context,
+    },
     # =========================================================================
     # Self-Contained Module Configuration
     # =========================================================================