Fix invitations: remove expires_at (invitations are permanent)

- Remove expires_at from SELECT queries
- Remove expiresAt from API responses
- DB change: dropped expires_at column, fixed FK to beneficiaries table

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

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

backend/src/routes/invitations.js

@@ -354,8 +354,7 @@ router.post('/', async (req, res) => {
         id: invitation.id,
         code: invitation.token,
         role: role,
-        email: email || null,
-        expiresAt: invitation.expires_at
+        email: email || null
       }
     });
 
@@ -389,7 +388,7 @@ router.get('/beneficiary/:beneficiaryId', async (req, res) => {
     // Get invitations for this beneficiary
     const { data: invitations, error } = await supabase
       .from('invitations')
-      .select('id, token, role, email, label, expires_at, accepted_at, accepted_by, created_at')
+      .select('id, token, role, email, label, accepted_at, accepted_by, created_at')
       .eq('beneficiary_id', beneficiaryId)
       .order('created_at', { ascending: false });
 
@@ -417,7 +416,6 @@ router.get('/beneficiary/:beneficiaryId', async (req, res) => {
         role: inv.role,
         email: inv.email,
         label: inv.label,
-        expiresAt: inv.expires_at,
         acceptedAt: inv.accepted_at,
         createdAt: inv.created_at,
         acceptedBy: acceptedUser ? {
@@ -561,7 +559,7 @@ router.get('/', async (req, res) => {
     // Get invitations
     const { data: invitations, error } = await supabase
       .from('invitations')
-      .select('id, token, role, email, label, beneficiary_id, expires_at, accepted_at, created_at')
+      .select('id, token, role, email, label, beneficiary_id, accepted_at, created_at')
       .eq('invited_by', userId)
       .order('created_at', { ascending: false });
 
@@ -585,7 +583,6 @@ router.get('/', async (req, res) => {
         role: inv.role,
         email: inv.email,
         label: inv.label,
-        expiresAt: inv.expires_at,
         acceptedAt: inv.accepted_at,
         createdAt: inv.created_at,
         beneficiary: beneficiary ? {