[mikahatdichlieb]

Closes #13

What

Uploads the local event buffer whenever the server is reachable.

• Triggers:

  • right away when the app is opened
  • hourly in the background while any network is connected
  • after a failure, with exponential backoff starting at 10 minutes

  There is no Wi-Fi detection, so the home network, Tailscale and public servers all work the same way. An unreachable server leaves the buffer for the next attempt.

• Each run collects new usage first, then uploads the buffer in batches of 500 events. 500 events stay far below 512 KiB.

• Events leave the buffer once the server accounts for them: accepted, duplicate, or rejected. For rejected events only the reason code and a count are logged.

• 429: the buffer is kept. No request is made from any path (app open, periodic run, retry) until Retry-After has passed, then a single delayed attempt runs.

• 401: the token is dropped and the server URL is kept, so the app shows "Pair this device again". No further device requests and no timer retry; the buffer and collection are kept.

• GET /api/v1/device/me: once per app start for the clock skew, never on a timer.

• Connected screen: says in words when the device last synced or why it could not, how many entries are waiting, and warns when the device clock is off by more than 5 minutes.

Other changes

• DeviceSetup is one instance per process, so pairing, sync and the 401 handling share its lock.
• "Server URL but no token" now means "pair again". A failed token save during pairing therefore removes the server URL too.
• The last successful sync time and the earliest allowed attempt after a 429 are stored in SharedPreferences, which are excluded from backups.

Security / privacy relevant

• After a 401 the token is deleted; nothing is retried on a timer.
• Logcat only ever contains reason codes and counts, never event data or tokens.
• Without Tailscale, each attempt away from home still does a DNS lookup of the server hostname on the current network.

Tests

EventSyncTest runs against an HTTPS MockWebServer and covers:

• no request without pairing
• accepted, duplicate and rejected events leave the buffer, and only reason codes are logged
• 501 events go out in two batches
• 401 keeps the buffer, asks to pair again and sends no further requests
• 429 with its delay, an unreachable server and a server error all keep the buffer
• an empty buffer sends no request
• /device/me clock skew and 401

The cross-path Retry-After gate and WorkManager scheduling depend on Android and are not unit tested. Nothing has been run on a device yet. There are no new dependencies and the Room schema is unchanged.