Add full-stack BTC portfolio web app

Multi-user FastAPI + React app with JWT auth, SQLite storage, and
CoinGecko price integration. Dockerized with docker-compose.

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

btc-portfolio/backend/app/services/btc.py

@@ -0,0 +1,14 @@
+import requests
+
+
+def get_btc_price_eur() -> float:
+    try:
+        resp = requests.get(
+            "https://api.coingecko.com/api/v3/simple/price",
+            params={"ids": "bitcoin", "vs_currencies": "eur"},
+            timeout=10,
+        )
+        resp.raise_for_status()
+        return float(resp.json()["bitcoin"]["eur"])
+    except Exception:
+        return 0.0