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/dependencies.py

@@ -27,3 +27,9 @@ def get_current_user(
     if user is None:
         raise HTTPException(status_code=status.HTTP_401_UNAUTHORIZED, detail="User not found")
     return user
+
+
+def get_current_admin(current_user: models.User = Depends(get_current_user)) -> models.User:
+    if not current_user.is_admin:
+        raise HTTPException(status_code=status.HTTP_403_FORBIDDEN, detail="Admin access required")
+    return current_user