feat: prevent duplicate membership applications and fees

- Add UNIQUE constraint on membership_application.user_id (one app per user)
- Add UNIQUE constraint on membership_fees.user_id (one fee record per user)
- Add validation checks in process_application.php before inserting
- Improve error messages for duplicate submission attempts
- Add migration script to clean up existing duplicates before constraints
- Update checkMembershipApplication to set session message on redirect
- Add comprehensive documentation of duplicate prevention architecture

Individual payments/EFTs are tracked separately in payments table

docs/migrations/002_add_unique_constraints_membership.sql

@@ -0,0 +1,37 @@
+-- Migration: Add UNIQUE constraints to prevent duplicate membership applications and fees
+-- Date: 2025-12-05
+-- Purpose: Ensure each user can only have one application and one membership fee record
+-- Note: Individual payments are tracked in the payments/efts table, not here
+
+-- Add UNIQUE constraint to membership_application table
+-- First, delete any duplicate applications keeping the most recent one
+DELETE FROM membership_application
+WHERE application_id NOT IN (
+    SELECT MAX(application_id) 
+    FROM (
+        SELECT application_id 
+        FROM membership_application
+    ) tmp
+    GROUP BY user_id
+);
+
+-- Add UNIQUE constraint on user_id in membership_application
+ALTER TABLE membership_application 
+ADD CONSTRAINT uk_membership_application_user_id UNIQUE (user_id);
+
+-- Add UNIQUE constraint to membership_fees table
+-- First, delete any duplicate fees keeping the most recent one
+DELETE FROM membership_fees
+WHERE fee_id NOT IN (
+    SELECT MAX(fee_id) 
+    FROM (
+        SELECT fee_id 
+        FROM membership_fees
+    ) tmp
+    GROUP BY user_id
+);
+
+-- Add UNIQUE constraint on user_id in membership_fees
+ALTER TABLE membership_fees 
+ADD CONSTRAINT uk_membership_fees_user_id UNIQUE (user_id);
+