Fix production deployment: replace serve with nginx reverse proxy

Frontend container now uses nginx to serve static files and proxy
/api/* requests to the backend container internally, eliminating
the hardcoded localhost:8000 build-time URL that caused "Network
error" on any non-local server. CORS origins are also configurable
via ALLOWED_ORIGINS env var.

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

btc-portfolio/backend/app/main.py

@@ -1,3 +1,5 @@
+import os
+
 from fastapi import FastAPI
 from fastapi.middleware.cors import CORSMiddleware
 from sqlalchemy import text
@@ -9,9 +11,15 @@ Base.metadata.create_all(bind=engine)
 
 app = FastAPI(title="BTC Portfolio API")
 
+_raw_origins = os.environ.get(
+    "ALLOWED_ORIGINS",
+    "http://localhost:3000,http://localhost:3001",
+)
+allowed_origins = [o.strip() for o in _raw_origins.split(",") if o.strip()]
+
 app.add_middleware(
     CORSMiddleware,
-    allow_origins=["http://localhost:3000", "http://localhost:3001"],
+    allow_origins=allowed_origins,
     allow_credentials=True,
     allow_methods=["GET", "POST", "PUT", "DELETE", "OPTIONS"],
     allow_headers=["Content-Type", "Authorization"],