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-agent/migrations/0005_users_and_play_events.sql

@@ -0,0 +1,20 @@
+CREATE TABLE users (
+    id           TEXT PRIMARY KEY,
+    username     TEXT NOT NULL,
+    display_name TEXT,
+    email        TEXT,
+    created_at   TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    last_seen_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+CREATE TABLE play_events (
+    id        BIGSERIAL PRIMARY KEY,
+    user_id   TEXT NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    track_id  BIGINT NOT NULL REFERENCES tracks(id) ON DELETE CASCADE,
+    played_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+CREATE INDEX idx_play_events_user_id ON play_events(user_id);
+CREATE INDEX idx_play_events_track_id ON play_events(track_id);
+CREATE INDEX idx_play_events_user_track ON play_events(user_id, track_id);
+CREATE INDEX idx_play_events_played_at ON play_events(played_at DESC);