security: round 1 — quick wins (rate limiting, headers, mass assignment, logging)

- Add throttle middleware to login (5/min), portal/token-auth (10/min),
  volunteer-register (5/min), and invitation routes (10/min)
- Set Sanctum token expiration to 7 days
- Remove billing_status from UpdateOrganisationRequest (super_admin only)
- Revoke all Sanctum tokens on password reset
- Strengthen password rules: min 8 chars, mixed case, numbers
- Create SecurityHeaders middleware (X-Content-Type-Options, X-Frame-Options,
  HSTS, Referrer-Policy, Permissions-Policy)
- Fix open redirect on all 3 login pages (validate ?to= starts with /)
- Set APP_DEBUG=false in .env.example
- Log failed login attempts with email, IP, user-agent
- Log authorization failures (403) with user, IP, path, method
- Harden mass assignment: remove user_id from Person, audit fields from
  ShiftAssignment, system fields from UserInvitation $fillable
- Replace real DB records with factory make() in mail preview routes

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

api/app/Models/ShiftAssignment.php

@@ -24,19 +24,9 @@ final class ShiftAssignment extends Model
         'person_id',
         'time_slot_id',
         'status',
-        'auto_approved',
-        'assigned_by',
-        'assigned_at',
-        'approved_by',
-        'approved_at',
         'rejection_reason',
-        'cancelled_by',
-        'cancellation_source',
-        'cancelled_at',
         'hours_expected',
         'hours_completed',
-        'checked_in_at',
-        'checked_out_at',
     ];
 
     protected function casts(): array