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>

backend/app/plan_limits.py

@@ -0,0 +1,49 @@
+"""Piano di abbonamento e limiti per feature."""
+
+PLAN_LIMITS = {
+    "freemium": {
+        "characters_max": 1,
+        "posts_per_month": 15,
+        "platforms": ["facebook", "instagram"],
+        "auto_plans": False,
+        "comments_management": False,
+        "affiliate_links": False,
+        "editorial_calendar_max": 5,
+    },
+    "pro": {
+        "characters_max": None,
+        "posts_per_month": None,
+        "platforms": ["facebook", "instagram", "youtube", "tiktok"],
+        "auto_plans": True,
+        "comments_management": True,
+        "affiliate_links": True,
+        "editorial_calendar_max": None,
+    },
+}
+
+
+def get_plan(user) -> dict:
+    """Returns the effective plan for a user (checks expiry for pro)."""
+    from datetime import datetime
+    plan = getattr(user, "subscription_plan", "freemium") or "freemium"
+    if plan == "pro":
+        expires = getattr(user, "subscription_expires_at", None)
+        if expires and expires < datetime.utcnow():
+            return PLAN_LIMITS["freemium"]
+    return PLAN_LIMITS.get(plan, PLAN_LIMITS["freemium"])
+
+
+def check_limit(user, feature: str, current_count: int = 0) -> tuple[bool, str]:
+    """Returns (allowed, error_message)."""
+    limits = get_plan(user)
+    limit = limits.get(feature)
+    if limit is None:
+        return True, ""
+    if isinstance(limit, bool):
+        if not limit:
+            return False, f"Feature '{feature}' non disponibile nel piano Freemium. Passa a Pro."
+        return True, ""
+    if isinstance(limit, int):
+        if current_count >= limit:
+            return False, f"Hai raggiunto il limite del piano Freemium ({limit} {feature}). Passa a Pro."
+    return True, ""