Sync the buffer to the server #13

Upload buffered events with POST /api/v1/events whenever the server is reachable.

• No Wi-Fi detection: try whenever any network is connected. That covers the home network, Tailscale and public servers alike. An unreachable server leaves the buffer untouched and is retried later.
• Triggers: periodic (hourly, network connected), a retry with backoff after a network failure, and right away when the app is opened.
• Batches of at most 500 events / 512 KiB. Drop events on accepted or duplicate; drop rejected ones and log only the reason code.
• 429: keep the batch, wait for Retry-After.
• 401: stop all device requests, keep the buffer, show "pair this device again". No retry on a timer.
• GET /api/v1/device/me once at app start for the clock skew, never polled.
• Events older than the server's window (90 days by default) are rejected by the server (start_ts_too_old) and dropped.