refactor: move capacity_forecast_service from billing to monitoring

Resolves the billing (core) → monitoring (optional) architecture violation
by moving CapacityForecastService to the monitoring module where it belongs.

- Create BillingMetricsProvider to expose subscription counts via stats_aggregator
- Move CapacitySnapshot model from billing to monitoring
- Replace direct MerchantSubscription queries with stats_aggregator calls
- Fix middleware test mocks to cover StoreDomain/MerchantDomain fallback chains

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/modules/billing/models/__init__.py

@@ -22,7 +22,6 @@ from app.modules.billing.models.subscription import (
     AddOnProduct,
     BillingHistory,
     BillingPeriod,
-    CapacitySnapshot,
     StoreAddOn,
     StripeWebhookEvent,
     SubscriptionStatus,
@@ -46,7 +45,6 @@ __all__ = [
     "StoreAddOn",
     "StripeWebhookEvent",
     "BillingHistory",
-    "CapacitySnapshot",
     # Merchant Subscription
     "MerchantSubscription",
     # Feature Limits

app/modules/billing/models/subscription.py

@@ -345,61 +345,3 @@ class BillingHistory(Base, TimestampMixin):
 
     def __repr__(self):
         return f"<BillingHistory(store_id={self.store_id}, invoice='{self.invoice_number}', status='{self.status}')>"
-
-
-# ============================================================================
-# Capacity Planning
-# ============================================================================
-
-
-class CapacitySnapshot(Base, TimestampMixin):
-    """
-    Daily snapshot of platform capacity metrics.
-
-    Used for growth trending and capacity forecasting.
-    Captured daily by background job.
-    """
-
-    __tablename__ = "capacity_snapshots"
-
-    id = Column(Integer, primary_key=True, index=True)
-    snapshot_date = Column(DateTime(timezone=True), nullable=False, unique=True, index=True)
-
-    # Store metrics
-    total_stores = Column(Integer, default=0, nullable=False)
-    active_stores = Column(Integer, default=0, nullable=False)
-    trial_stores = Column(Integer, default=0, nullable=False)
-
-    # Subscription metrics
-    total_subscriptions = Column(Integer, default=0, nullable=False)
-    active_subscriptions = Column(Integer, default=0, nullable=False)
-
-    # Resource metrics
-    total_products = Column(Integer, default=0, nullable=False)
-    total_orders_month = Column(Integer, default=0, nullable=False)
-    total_team_members = Column(Integer, default=0, nullable=False)
-
-    # Storage metrics
-    storage_used_gb = Column(Numeric(10, 2), default=0, nullable=False)
-    db_size_mb = Column(Numeric(10, 2), default=0, nullable=False)
-
-    # Capacity metrics (theoretical limits from subscriptions)
-    theoretical_products_limit = Column(Integer, nullable=True)
-    theoretical_orders_limit = Column(Integer, nullable=True)
-    theoretical_team_limit = Column(Integer, nullable=True)
-
-    # Tier distribution (JSON: {"essential": 10, "professional": 5, ...})
-    tier_distribution = Column(JSON, nullable=True)
-
-    # Performance metrics
-    avg_response_ms = Column(Integer, nullable=True)
-    peak_cpu_percent = Column(Numeric(5, 2), nullable=True)
-    peak_memory_percent = Column(Numeric(5, 2), nullable=True)
-
-    # Indexes
-    __table_args__ = (
-        Index("ix_capacity_snapshots_date", "snapshot_date"),
-    )
-
-    def __repr__(self) -> str:
-        return f"<CapacitySnapshot(date={self.snapshot_date}, stores={self.total_stores})>"