feat(setup): add a setup token for first-run setup (#3437)

deploy/self-hosting/.env.example

@@ -344,6 +344,8 @@ FLUXER_DISCOVERY_ENABLED=true
 #FLUXER_APP_STATUS_PAGE_URL=
 #FLUXER_APP_STATUS_PAGE_INCIDENT_HISTORY_URL=
 #FLUXER_INSTANCE_SETUP_CONFIGURED=false
+# Setup token the wizard asks for before the first admin account. 32 to 256 characters. Unset means the API generates one on the first start and writes it to its log once.
+#FLUXER_SETUP_TOKEN=
 # How members sign in on a new instance, username or email. Unset means username. Read only on the first start.
 #FLUXER_ACCOUNT_IDENTITY=
 # Username tags on a new email instance. none gives unique names with no tag, random gives name#4821. Unset means none. A username instance always uses none. Read only on the first start.
@@ -536,7 +538,7 @@ FLUXER_DISCOVERY_ENABLED=true
 
 # The bundled Valkey holds durable state as well as cache, so it runs with an
 # append-only file and with noeviction, which fails an over-limit write instead
-# of dropping queued work. Change the policy only if that state lives elsewhere.
+# of dropping queued work. Change the policy only if that state is kept elsewhere.
 #FLUXER_VALKEY_MAXMEMORY=192mb
 #FLUXER_VALKEY_MAXMEMORY_POLICY=noeviction
 #FLUXER_VALKEY_APPENDFSYNC=everysec