feat: add user support with play event tracking

Backend (Rust API):
- Add users and play_events tables (migration 0005)
- Extract full user identity from JWT (sub, username, email, name)
  and pass AuthUser via request extensions to all handlers
- Auto-upsert user in background on every authenticated request
- POST /api/tracks/:slug/play endpoint to record play events
- Allow POST method in CORS

Frontend (Node player):
- Call recordPlay() when a track starts playing
- Add user profile avatar with dropdown menu (name, email, sign out)
- Pass user info from App through FurumiPlayer to Header

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

furumi-node-player/client/src/components/header/header.module.css

@@ -32,4 +32,80 @@
     margin-left: 0.25rem;
     font-weight: 500;
     text-decoration: none;
+}
+
+/* User menu */
+
+.userMenu {
+    position: relative;
+}
+
+.userAvatar {
+    width: 32px;
+    height: 32px;
+    border-radius: 50%;
+    background: var(--accent);
+    color: #fff;
+    border: none;
+    font-size: 0.75rem;
+    font-weight: 600;
+    cursor: pointer;
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    transition: background 0.2s;
+}
+
+.userAvatar:hover {
+    background: var(--accent-dim);
+}
+
+.userDropdown {
+    position: absolute;
+    top: calc(100% + 8px);
+    right: 0;
+    min-width: 200px;
+    background: var(--bg-card);
+    border: 1px solid var(--border);
+    border-radius: 10px;
+    box-shadow: 0 12px 32px rgba(0, 0, 0, 0.4);
+    z-index: 100;
+    overflow: hidden;
+    animation: fadeIn 0.15s ease;
+}
+
+@keyframes fadeIn {
+    from { opacity: 0; transform: translateY(-4px); }
+    to { opacity: 1; transform: translateY(0); }
+}
+
+.userInfo {
+    padding: 12px 16px;
+    border-bottom: 1px solid var(--border);
+}
+
+.userName {
+    font-weight: 600;
+    font-size: 0.9rem;
+    color: var(--text);
+}
+
+.userEmail {
+    font-size: 0.75rem;
+    color: var(--text-muted);
+    margin-top: 2px;
+}
+
+.userLogout {
+    display: block;
+    padding: 10px 16px;
+    color: var(--danger);
+    text-decoration: none;
+    font-size: 0.85rem;
+    font-weight: 500;
+    transition: background 0.15s;
+}
+
+.userLogout:hover {
+    background: var(--bg-hover);
 }