fix: additional code quality and performance improvements

Code quality:
- Remove empty except blocks with proper logging
- Create shared pagination utility function
- Remove duplicate UUID validation code
- Fix dead code in translation.py

Performance:
- Fix N+1 query in followup engine (use join instead of loop)
- Add eager loading for customer health scores
- Create database indexes for common query patterns:
  - customers: (user_id, status), (user_id, last_contact_at)
  - payment_transactions: (user_id, created_at)
  - followup_logs: (user_id, customer_id)
  - notifications: (user_id, is_read)

Configuration:
- Centralize magic numbers in config.py:
  - Payment prices
  - File upload limits
  - Rate limiting settings
  - Pagination defaults
- Update auth.py to use centralized rate limiting config
- Update customer/product imports to use centralized upload limits
- Update import_service.py to use centralized MAX_ROWS

backend/alembic/versions/add_performance_indexes.py

@@ -0,0 +1,41 @@
+"""add performance indexes
+
+Revision ID: add_perf_indexes
+Revises: add_payment_transactions_table
+Create Date: 2026-06-11
+
+Add indexes for common query patterns to improve performance.
+"""
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision = 'add_perf_indexes'
+down_revision = 'add_payment_transactions_table'
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    # Customer indexes
+    op.create_index('ix_customer_user_status', 'customers', ['user_id', 'status'])
+    op.create_index('ix_customer_user_last_contact', 'customers', ['user_id', 'last_contact_at'])
+    
+    # Payment transaction indexes
+    op.create_index('ix_payment_user_created', 'payment_transactions', ['user_id', 'created_at'])
+    
+    # Followup log indexes
+    op.create_index('ix_followup_user_customer', 'followup_logs', ['user_id', 'customer_id'])
+    
+    # Notification indexes
+    op.create_index('ix_notification_user_read', 'notifications', ['user_id', 'is_read'])
+
+
+def downgrade() -> None:
+    # Remove indexes
+    op.drop_index('ix_notification_user_read', 'notifications')
+    op.drop_index('ix_followup_user_customer', 'followup_logs')
+    op.drop_index('ix_payment_user_created', 'payment_transactions')
+    op.drop_index('ix_customer_user_last_contact', 'customers')
+    op.drop_index('ix_customer_user_status', 'customers')