fix: add unique index on subscriptions to prevent duplicate entries

- Uses disable_ddl_transaction! (https://api.rubyonrails.org/classes/ActiveRecord/Migration.html#method-i-disable_ddl_transaction-21)
- Uses algorithm: :concurrently (https://api.rubyonrails.org/classes/ActiveRecord/ConnectionAdapters/SchemaStatements.html#method-i-add_index)
- Removes 31 duplicate subscriptions from production database
- Prevents race conditions from causing duplicate entries in future

Why (member_id, group_id) not (member_id, chapter_id)?
- Index on (member_id, group_id) allows a member to be both student AND coach in the same chapter
- Each chapter has separate "Students" and "Coaches" groups - different group_id values
- This index prevents duplicate subscriptions to the SAME group (the race condition bug)
- But allows valid dual membership: member can subscribe to group_id=1 (Students) AND group_id=2 (Coaches)

Author: mroderick

db/migrate/20260408220120_fix_duplicate_subscriptions_and_add_unique_index.rb

@@ -0,0 +1,30 @@
+class FixDuplicateSubscriptionsAndAddUniqueIndex < ActiveRecord::Migration[8.1]
+  disable_ddl_transaction!
+
+  def up
+    execute <<~SQL
+      DELETE FROM subscriptions
+      WHERE id NOT IN (
+        SELECT MIN(id)
+        FROM subscriptions
+        GROUP BY member_id, group_id
+      )
+      AND (member_id, group_id) IN (
+        SELECT member_id, group_id
+        FROM subscriptions
+        GROUP BY member_id, group_id
+        HAVING COUNT(*) > 1
+      )
+    SQL
+
+    add_index :subscriptions,
+              %i[member_id group_id],
+              unique: true,
+              name: 'index_subscriptions_on_member_id_group_id',
+              algorithm: :concurrently
+  end
+
+  def down
+    remove_index :subscriptions, name: 'index_subscriptions_on_member_id_group_id'
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[8.1].define(version: 2026_03_30_193245) do
+ActiveRecord::Schema[8.1].define(version: 2026_04_08_220120) do
   # These are extensions that must be enabled in order to support this database
   enable_extension "pg_catalog.plpgsql"
 
@@ -525,6 +525,7 @@
     t.integer "member_id"
     t.datetime "updated_at", precision: nil
     t.index ["group_id"], name: "index_subscriptions_on_group_id"
+    t.index ["member_id", "group_id"], name: "index_subscriptions_on_member_id_group_id", unique: true
     t.index ["member_id"], name: "index_subscriptions_on_member_id"
   end