feat: multi-user SaaS, piani Freemium/Pro, Google OAuth, admin panel

BLOCCO 1 - Multi-user data model:
- User: email, display_name, avatar_url, auth_provider, google_id
- User: subscription_plan, subscription_expires_at, is_admin, post counters
- SubscriptionCode table per redeem codes
- user_id FK su Character, Post, AffiliateLink, EditorialPlan, SocialAccount, SystemSetting
- Migrazione SQLite-safe (ALTER TABLE) + preserva dati esistenti

BLOCCO 2 - Auth completo:
- Registrazione email/password + login multi-user
- Google OAuth 2.0 (httpx, no deps esterne)
- Callback flow: Google -> /auth/callback?token=JWT -> frontend
- Backward compat login admin con username

BLOCCO 3 - Piani e abbonamenti:
- Freemium: 1 character, 15 post/mese, FB+IG only, no auto-plans
- Pro: illimitato, tutte le piattaforme, tutte le feature
- Enforcement automatico in tutti i router
- Redeem codes con durate 1/3/6/12 mesi
- Admin panel: genera codici, lista utenti

BLOCCO 4 - Frontend completo:
- Login page design Leopost (split coral/cream, Google, social coming soon)
- AuthCallback per OAuth redirect
- PlanBanner, UpgradeModal con pricing
- AdminSettings per generazione codici
- CharacterForm con tab Account Social + guide setup

Deploy:
- Dockerfile con ARG VITE_BASE_PATH/VITE_API_BASE
- docker-compose.prod.yml per leopost.it (no subpath)
- docker-compose.yml aggiornato per lab

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Dockerfile

@@ -4,6 +4,10 @@ WORKDIR /app/frontend
 COPY frontend/package*.json ./
 RUN npm install
 COPY frontend/ ./
+ARG VITE_BASE_PATH=/leopost-full
+ARG VITE_API_BASE=/leopost-full/api
+ENV VITE_BASE_PATH=$VITE_BASE_PATH
+ENV VITE_API_BASE=$VITE_API_BASE
 RUN npm run build
 
 # Stage 2: Python backend + frontend built