feat(loyalty): refactor analytics into shared template and add merchant stats API

Extract analytics stat cards, points activity, and location breakdown
into a shared partial used by admin, merchant, and store dashboards.
Add merchant stats API endpoint and client-side merchant filter on admin
analytics page. Extend stats schema with new_this_month and
estimated_liability_cents fields.

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

app/modules/loyalty/schemas/program.py

@@ -284,11 +284,17 @@ class MerchantStatsResponse(BaseModel):
     total_points_issued: int = 0
     total_points_redeemed: int = 0
 
+    # Members
+    new_this_month: int = 0
+
     # Points - last 30 days
     points_issued_30d: int = 0
     points_redeemed_30d: int = 0
     transactions_30d: int = 0
 
+    # Value
+    estimated_liability_cents: int = 0
+
     # Program info (optional)
     program: dict | None = None