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/main.py

@@ -1,8 +1,9 @@
 from fastapi import FastAPI
 from fastapi.middleware.cors import CORSMiddleware
+from sqlalchemy import text
 
 from .database import engine, Base
-from .routes import users, purchases, stats, history
+from .routes import users, purchases, stats, history, admin
 
 Base.metadata.create_all(bind=engine)
 
@@ -20,6 +21,16 @@ app.include_router(users.router)
 app.include_router(purchases.router)
 app.include_router(stats.router)
 app.include_router(history.router)
+app.include_router(admin.router, prefix="/admin")
+
+
+@app.on_event("startup")
+def migrate():
+    with engine.connect() as conn:
+        cols = [r[1] for r in conn.execute(text("PRAGMA table_info(users)"))]
+        if "is_admin" not in cols:
+            conn.execute(text("ALTER TABLE users ADD COLUMN is_admin BOOLEAN DEFAULT 0"))
+            conn.commit()
 
 
 @app.get("/")