Add super admin panel and update README

- Admin account bootstrapped from ADMIN_EMAIL/ADMIN_PASSWORD env vars on startup
- Admin panel: list users, view content, reset passwords, disable/delete accounts
- is_admin and is_disabled columns on users table
- Disabled accounts blocked at login
- README updated with admin setup instructions and panel docs

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

backend/models.py

@@ -41,6 +41,8 @@ class User(Base):
     id = Column(Integer, primary_key=True, autoincrement=True)
     email = Column(String(255), unique=True, nullable=False)
     hashed_password = Column(String(255), nullable=False)
+    is_admin = Column(Boolean, default=False, nullable=False)
+    is_disabled = Column(Boolean, default=False, nullable=False)
     created_at = Column(DateTime, server_default=func.now())
 
     varieties = relationship("Variety", back_populates="user", cascade="all, delete-orphan")