Fix a systemd service stuck in start-limit-hit ('Start request repeated too quickly')

Documented steps

1. Symptom in journal/status: `myapp.service: Start request repeated too quickly.` followed by `Failed to start ...` and `Active: failed (Result: start-limit-hit)`.
2. Cause: the unit was (re)started more than StartLimitBurst times (default 5) within StartLimitIntervalSec (default 10s). With Restart=always and a fast-crashing process this triggers within a second.
3. Clear the state immediately so manual starts work again: `systemctl reset-failed myapp.service`, then `systemctl start myapp.service`. Fix the underlying crash using journalctl before touching limits.
4. To make restarts sustainable, slow them down rather than removing the limit: in [Service] set `RestartSec=5` (default is 100ms).
5. To widen the rate limit, set in the [Unit] section (NOT [Service]): [Unit] StartLimitIntervalSec=300 StartLimitBurst=10
6. For a service that should retry forever without ever hitting the limit, ensure RestartSec * StartLimitBurst > StartLimitIntervalSec, or set StartLimitIntervalSec=0 to disable rate limiting entirely.
7. Apply with `systemctl daemon-reload && systemctl restart myapp.service`.