feat: person identity matching with detection, confirmation and audit trail

Implements enterprise-grade identity resolution (detect → suggest → confirm)
for Person ↔ User linking. Matches are detected automatically on person
creation and user account creation, then surfaced to organisers for explicit
confirmation or dismissal. No silent auto-linking.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

api/app/Models/User.php

@@ -64,6 +64,11 @@ final class User extends Authenticatable
         return $this->hasMany(UserInvitation::class, 'invited_by_user_id');
     }
 
+    public function identityMatches(): HasMany
+    {
+        return $this->hasMany(PersonIdentityMatch::class, 'matched_user_id');
+    }
+
     public function organisationTags(): HasMany
     {
         return $this->hasMany(UserOrganisationTag::class);