Add admin role with user management (create/delete users)

First registered user becomes admin automatically. Admins see a
"Manage Users" button in the dashboard header that opens a new
/admin page for listing, creating, and deleting users. Backend
enforces admin-only access on /admin/* routes. Startup migration
adds the is_admin column to existing SQLite databases.

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

btc-portfolio/backend/app/models.py

@@ -1,4 +1,4 @@
-from sqlalchemy import Column, Integer, String, Float, ForeignKey, DateTime
+from sqlalchemy import Column, Integer, String, Float, ForeignKey, DateTime, Boolean
 from sqlalchemy.orm import relationship
 from datetime import datetime
 from .database import Base
@@ -10,6 +10,7 @@ class User(Base):
     id = Column(Integer, primary_key=True, index=True)
     username = Column(String, unique=True, index=True, nullable=False)
     password = Column(String, nullable=False)
+    is_admin = Column(Boolean, default=False, nullable=False, server_default='0')
 
     purchases = relationship("Purchase", back_populates="owner", cascade="all, delete")