feat: automatically remove queued messages with stale locks (#2872)

doc/config/environment-variables.md

@@ -18,6 +18,7 @@ This document contains all the environment variables which are available for thi
 | `POSTAL_SIGNING_KEY_PATH` | String | Path to the private key used for signing | $config-file-root/signing.key |
 | `POSTAL_SMTP_RELAYS` | Array of strings | An array of SMTP relays in the format of smtp://host:port | [] |
 | `POSTAL_TRUSTED_PROXIES` | Array of strings | An array of IP addresses to trust for proxying requests to Postal (in addition to localhost addresses) | [] |
+| `POSTAL_QUEUED_MESSAGE_LOCK_STALE_DAYS` | Integer | The number of days after which to consider a lock as stale. Messages with stale locks will be removed and not retried. | 1 |
 | `WEB_SERVER_DEFAULT_PORT` | Integer | The default port the web server should listen on unless overriden by the PORT environment variable | 5000 |
 | `WEB_SERVER_DEFAULT_BIND_ADDRESS` | String | The default bind address the web server should listen on unless overriden by the BIND_ADDRESS environment variable | 127.0.0.1 |
 | `WEB_SERVER_MAX_THREADS` | Integer | The maximum number of threads which can be used by the web server | 5 |