dovecot: migrated config to 2.4 + config splitting

data/conf/dovecot/conf.d/20-auth.conf

@@ -0,0 +1,40 @@
+# /etc/dovecot/conf.d/20-auth.conf
+# Authentication mechanisms, master/user separation, passdb chain, auth cache.
+auth_mechanisms = plain login
+auth_allow_cleartext = yes
+auth_master_user_separator = *
+
+auth_cache_verify_password_with_worker = yes
+auth_cache_negative_ttl = 60s
+auth_cache_ttl = 300s
+auth_cache_size = 10M
+auth_verbose_passwords = sha1:6
+
+# 1) Lua password verification (blocking, return mapping).
+passdb lua {
+  driver = lua
+  lua_file = /etc/dovecot/auth/passwd-verify.lua
+  lua_settings {
+    blocking=yes
+    result_success = return-ok
+    result_failure = continue
+    result_internalfail = continue
+  }
+}
+
+# 2) Master password for master user logins.
+passdb master {
+  driver = passwd-file
+  passwd_file_path = /etc/dovecot/dovecot-master.passwd 
+  master = yes
+  skip = authenticated
+}
+
+# 3) Mandatory return layer: empty Lua (e.g. for forced reset).
+passdb empty-lua {
+  driver = lua
+  lua_file = /etc/dovecot/auth/passwd-verify.lua
+  lua_settings {
+    blocking = yes
+  }
+}