feat: complete person identity matching system with fuzzy detection, revert, and manual link

Implements the full identity matching engine: email matching (HIGH confidence),
fuzzy name matching with Levenshtein distance (MEDIUM confidence, upgradable to
HIGH with DOB tiebreaker), manual link/unlink, revert confirmed matches, and
automatic detection via PersonObserver. Includes 33 comprehensive tests, frontend
integration with confirm/dismiss/unlink UI, and match indicators in the persons list.

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

api/app/Models/User.php

@@ -26,6 +26,7 @@ final class User extends Authenticatable
     protected $fillable = [
         'first_name',
         'last_name',
+        'date_of_birth',
         'email',
         'password',
         'timezone',
@@ -51,6 +52,7 @@ final class User extends Authenticatable
     protected function casts(): array
     {
         return [
+            'date_of_birth' => 'date',
             'email_verified_at' => 'datetime',
             'password' => 'hashed',
         ];