Security hardening: secrets, validation, Docker, and error handling

- Add root .gitignore to prevent btc_wallet.py (with RPC credentials) from being committed
- Load JWT SECRET_KEY from environment variable instead of hardcoded value
- Restrict CORS to explicit methods/headers instead of wildcards
- Add Pydantic Field validation (gt=0) to purchase amounts and user credentials
- Add logging to all silent exception handlers in btc.py
- Run backend and frontend Docker containers as non-root appuser
- Add .dockerignore for both backend and frontend
- Pass SECRET_KEY env var through docker-compose; add healthchecks to both services
- Update bcrypt from pinned 3.2.2 to >=4.0.0
- Capture error objects in frontend catch blocks; check admin delete response

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

btc-portfolio/frontend/src/pages/AdminPage.js

@@ -66,7 +66,12 @@ export default function AdminPage() {
 
   const handleDelete = async (id, name) => {
     if (!window.confirm(`Delete user "${name}"? This also deletes all their purchases.`)) return;
-    await fetch(`${API}/admin/users/${id}`, { method: 'DELETE', headers: authHeaders() });
+    const res = await fetch(`${API}/admin/users/${id}`, { method: 'DELETE', headers: authHeaders() });
+    if (!res.ok) {
+      const data = await res.json().catch(() => ({}));
+      setError(data.detail || 'Failed to delete user');
+      return;
+    }
     fetchUsers();
   };