Reduce repeated logins to Mergin Maps server

[varmar05]

Issue

Sometimes there are db-syncs logging in very frequently (e.g. every ~2 s). By design, db-sync logs in once per process, so the cause is processes being started over and over: a failing daemon restarted by Docker or another service manager, or --single-run called frequently from cron. Each new process logged in again.

Fixes

1. Retry failed start in-process instead of exiting

• Login, --force-init cleaning and init failures are retried inside the daemon, with exponential backoff from sleep_time up to 10 min. An existing client is reused, so a failed init doesn't log in again.
• Unexpected exceptions are caught too, not only DbSyncError.
• New daemon.max_retries option (default 10, 0 = never exit). The daemon exits after this many consecutive failed retries of the start or of unexpected errors, and always sends a notification email when it gives up. Sync errors after a successful start are still retried indefinitely.

2. Reuse auth token across restarts

• The token is stored in <working_dir>/.mergin_auth.json.
• On start it's reused if it was issued for the configured server and user, has more than 1 h of validity left and the server still accepts it. Otherwise db-sync logs in normally.
• The token survives --force-init cleaning.

3. Handle tokens rejected by the server

• If the server rejects the token of a running daemon (401), the stored token is dropped and db-sync logs in on the next retry. Previously sync failed until the token was close to expiring.
• Failed logins back off up to 1 h instead of 10 min, because rejected credentials can't be fixed by retrying and repeated failures may lock the account.