Demo plan + Payment integrations (Stripe/PayPal/Bank)

Models & migrations:
- platform_settings table (key/value JSON store + Cache::remember 5min)
- plans: is_demo bool + trial_days int
- companies: is_demo bool

Plans:
- Demo plan seeded (is_demo=true, is_public=false, all features, 14 trial days)
- Trial 14-day plan seeded (is_public=true, basic features)
- Plan form: is_demo toggle + trial_days field
- Plan table: badge 🎬 Demo / 🎁 N zile trial

Central panel:
- PaymentSettings page (heroicon-credit-card, sort 90)
  Form sections: General, Date legale, Stripe, PayPal, Transfer bancar
  Each gateway collapsible, fields hidden until enabled toggle
  Saves to platform_settings keyed by `payments.{gateway}`
- CompanyResource: is_demo toggle + table description

Payment flow (PaymentController):
- GET  /billing                 — tenant invoices list with Pay button
- POST /pay/{sub}               — start checkout (stripe/paypal/bank)
- GET  /pay/{sub}/{success,cancel}
- POST /payments/stripe/webhook — mark paid + extend company.active_until
- POST /payments/paypal/webhook — same

Views:
- site/billing.blade.php       — invoices list with payment modal (3 methods)
- site/bank-instructions       — IBAN/BIC/reference for manual transfer
- site/checkout-stub           — placeholder until composer require stripe-php
- site/payment-{success,cancel}

Tenant panel:
- userMenuItems → "Facturile mele" link to /billing

app/Filament/Central/Resources/CompanyResource.php

@@ -81,6 +81,9 @@ class CompanyResource extends Resource
                         ->searchable(),
                     Forms\Components\DateTimePicker::make('trial_ends_at')->label('Trial expiră la'),
                     Forms\Components\DateTimePicker::make('active_until')->label('Abonament până la'),
+                    Forms\Components\Toggle::make('is_demo')
+                        ->label('Tenant demo')
+                        ->helperText('Marchează acest tenant ca demo (datele pot fi resetate periodic).'),
                 ]),
             Schemas\Components\Section::make('Admin tenant (la creare)')
                 ->columns(2)
@@ -112,7 +115,9 @@ class CompanyResource extends Resource
                     ->copyable()
                     ->url(fn (Company $record) => $record->url('/app'))
                     ->openUrlInNewTab(),
-                Tables\Columns\TextColumn::make('name')->searchable()->sortable(),
+                Tables\Columns\TextColumn::make('name')
+                    ->searchable()->sortable()
+                    ->description(fn ($record) => $record->is_demo ? '🎬 demo' : null),
                 Tables\Columns\TextColumn::make('status')
                     ->badge()
                     ->colors([