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/database.py

@@ -1,4 +1,4 @@
-from sqlalchemy import create_engine
+from sqlalchemy import create_engine, text
 from sqlalchemy.orm import sessionmaker, declarative_base
 
 from .config import settings
@@ -18,3 +18,69 @@ def get_db():
         yield db
     finally:
         db.close()
+
+
+def run_migrations(engine):
+    """SQLite-safe migration: add new columns if they don't exist."""
+    migrations = {
+        "users": [
+            ("email", "VARCHAR"),
+            ("display_name", "VARCHAR"),
+            ("avatar_url", "VARCHAR"),
+            ("auth_provider", "VARCHAR DEFAULT 'local'"),
+            ("google_id", "VARCHAR"),
+            ("subscription_plan", "VARCHAR DEFAULT 'freemium'"),
+            ("subscription_expires_at", "DATETIME"),
+            ("is_admin", "BOOLEAN DEFAULT 0"),
+            ("posts_generated_this_month", "INTEGER DEFAULT 0"),
+            ("posts_reset_date", "DATE"),
+        ],
+        "characters": [("user_id", "INTEGER")],
+        "posts": [("user_id", "INTEGER")],
+        "affiliate_links": [("user_id", "INTEGER")],
+        "editorial_plans": [("user_id", "INTEGER")],
+        "social_accounts": [("user_id", "INTEGER")],
+        "system_settings": [("user_id", "INTEGER")],
+    }
+    with engine.connect() as conn:
+        for table, cols in migrations.items():
+            try:
+                existing = {row[1] for row in conn.execute(text(f"PRAGMA table_info({table})"))}
+                for col_name, col_def in cols:
+                    if col_name not in existing:
+                        conn.execute(text(f"ALTER TABLE {table} ADD COLUMN {col_name} {col_def}"))
+                conn.commit()
+            except Exception as e:
+                print(f"Migration warning for {table}: {e}")
+
+        # Fix system_settings: remove UNIQUE constraint on 'key' by recreating the table
+        # This allows per-user settings (same key, different user_id)
+        try:
+            indexes = list(conn.execute(text("PRAGMA index_list(system_settings)")))
+            has_unique_key = any(
+                row[1].lower().startswith("ix_") or "key" in row[1].lower()
+                for row in indexes
+                if row[2] == 1  # unique=1
+            )
+            # Check via table creation SQL
+            create_sql_row = conn.execute(text(
+                "SELECT sql FROM sqlite_master WHERE type='table' AND name='system_settings'"
+            )).fetchone()
+            if create_sql_row and "UNIQUE" in (create_sql_row[0] or "").upper():
+                # Recreate without UNIQUE on key
+                conn.execute(text("ALTER TABLE system_settings RENAME TO system_settings_old"))
+                conn.execute(text("""
+                    CREATE TABLE system_settings (
+                        id INTEGER PRIMARY KEY,
+                        key VARCHAR(100) NOT NULL,
+                        value JSON,
+                        updated_at DATETIME,
+                        user_id INTEGER REFERENCES users(id)
+                    )
+                """))
+                conn.execute(text("INSERT INTO system_settings SELECT id, key, value, updated_at, user_id FROM system_settings_old"))
+                conn.execute(text("DROP TABLE system_settings_old"))
+                conn.commit()
+                print("Migration: system_settings UNIQUE constraint on key removed.")
+        except Exception as e:
+            print(f"Migration warning for system_settings UNIQUE fix: {e}")