feat: complete subscription billing system phases 6-10

Phase 6 - Database-driven tiers:
- Update subscription_service to query database first with legacy fallback
- Add get_tier_info() db parameter and _get_tier_from_legacy() method

Phase 7 - Platform health integration:
- Add get_subscription_capacity() for theoretical vs actual capacity
- Include subscription capacity in full health report

Phase 8 - Background subscription tasks:
- Add reset_period_counters() for billing period resets
- Add check_trial_expirations() for trial management
- Add sync_stripe_status() for Stripe synchronization
- Add cleanup_stale_subscriptions() for maintenance
- Add capture_capacity_snapshot() for daily metrics

Phase 10 - Capacity planning & forecasting:
- Add CapacitySnapshot model for historical tracking
- Create capacity_forecast_service with growth trends
- Add /subscription-capacity, /trends, /recommendations endpoints
- Add /snapshot endpoint for manual captures

Also includes billing API enhancements from phase 4:
- Add upcoming-invoice, change-tier, addon purchase/cancel endpoints
- Add UsageSummary schema for billing page
- Enhance billing.js with addon management functions

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

alembic/versions/l0a1b2c3d4e5_add_capacity_snapshots_table.py

@@ -0,0 +1,65 @@
+"""Add capacity_snapshots table
+
+Revision ID: l0a1b2c3d4e5
+Revises: k9f0a1b2c3d4
+Create Date: 2025-12-26
+
+Adds table for tracking daily platform capacity metrics for growth forecasting.
+"""
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision = "l0a1b2c3d4e5"
+down_revision = "k9f0a1b2c3d4"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "capacity_snapshots",
+        sa.Column("id", sa.Integer(), nullable=False),
+        sa.Column("snapshot_date", sa.DateTime(timezone=True), nullable=False),
+        # Vendor metrics
+        sa.Column("total_vendors", sa.Integer(), nullable=False, server_default="0"),
+        sa.Column("active_vendors", sa.Integer(), nullable=False, server_default="0"),
+        sa.Column("trial_vendors", sa.Integer(), nullable=False, server_default="0"),
+        # Subscription metrics
+        sa.Column("total_subscriptions", sa.Integer(), nullable=False, server_default="0"),
+        sa.Column("active_subscriptions", sa.Integer(), nullable=False, server_default="0"),
+        # Resource metrics
+        sa.Column("total_products", sa.Integer(), nullable=False, server_default="0"),
+        sa.Column("total_orders_month", sa.Integer(), nullable=False, server_default="0"),
+        sa.Column("total_team_members", sa.Integer(), nullable=False, server_default="0"),
+        # Storage metrics
+        sa.Column("storage_used_gb", sa.Numeric(10, 2), nullable=False, server_default="0"),
+        sa.Column("db_size_mb", sa.Numeric(10, 2), nullable=False, server_default="0"),
+        # Capacity metrics
+        sa.Column("theoretical_products_limit", sa.Integer(), nullable=True),
+        sa.Column("theoretical_orders_limit", sa.Integer(), nullable=True),
+        sa.Column("theoretical_team_limit", sa.Integer(), nullable=True),
+        # Tier distribution
+        sa.Column("tier_distribution", sa.JSON(), nullable=True),
+        # Performance metrics
+        sa.Column("avg_response_ms", sa.Integer(), nullable=True),
+        sa.Column("peak_cpu_percent", sa.Numeric(5, 2), nullable=True),
+        sa.Column("peak_memory_percent", sa.Numeric(5, 2), nullable=True),
+        # Timestamps
+        sa.Column("created_at", sa.DateTime(timezone=True), server_default=sa.func.now(), nullable=False),
+        sa.Column("updated_at", sa.DateTime(timezone=True), server_default=sa.func.now(), nullable=False),
+        # Primary key
+        sa.PrimaryKeyConstraint("id"),
+    )
+
+    # Create indexes
+    op.create_index("ix_capacity_snapshots_id", "capacity_snapshots", ["id"], unique=False)
+    op.create_index("ix_capacity_snapshots_date", "capacity_snapshots", ["snapshot_date"], unique=True)
+
+
+def downgrade() -> None:
+    op.drop_index("ix_capacity_snapshots_date", table_name="capacity_snapshots")
+    op.drop_index("ix_capacity_snapshots_id", table_name="capacity_snapshots")
+    op.drop_table("capacity_snapshots")