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>

docker-compose.prod.yml

@@ -0,0 +1,29 @@
+services:
+  app:
+    build:
+      context: .
+      dockerfile: Dockerfile
+      args:
+        VITE_BASE_PATH: ""
+        VITE_API_BASE: "/api"
+    container_name: prod-leopost-full-app
+    restart: unless-stopped
+    volumes:
+      - ./data:/app/data
+    environment:
+      - DATABASE_URL=sqlite:///./data/leopost.db
+      - APP_URL=https://leopost.it
+      - GOOGLE_CLIENT_ID=${GOOGLE_CLIENT_ID}
+      - GOOGLE_CLIENT_SECRET=${GOOGLE_CLIENT_SECRET}
+      - SECRET_KEY=${SECRET_KEY:-leopost-prod-secret-2026}
+    networks:
+      - proxy_net
+    deploy:
+      resources:
+        limits:
+          memory: 1024M
+          cpus: '1.0'
+
+networks:
+  proxy_net:
+    external: true