fix: relativize zone-apex hostnames to '@' for CoreDNS MySQL 🐛

CoreDNS MySQL (cybercinch fork) expects '@' for zone-apex references in
record RDATA. Storing the full FQDN (e.g. 'ithome.net.nz.') caused CoreDNS
to strip the zone suffix and serve 'MX 0 .' / 'CNAME .' instead of the
correct apex target.

- Add _relativize_name(): converts zone FQDN → '@', in-zone subdomains →
  relative label, external FQDNs left unchanged. Handles both already-
  relativized output from dnspython ($ORIGIN present) and absolute FQDNs
  when $ORIGIN is absent from the zone file.
- Replace _normalize_cname_data() with _relativize_name(); add
  _normalize_mx_data(), _normalize_ns_data(), _normalize_srv_data() using
  the same helper.
- _parse_zone_to_record_set() now normalizes MX, NS, SRV alongside CNAME.
- _ensure_zone_exists() sets managed_by='directadmin' on create and
  back-fills NULL rows from pre-migration installs.
- Zone.managed_by changed to nullable=True to match ALTER TABLE migration
  where existing rows have no value.
- schema/coredns_mysql.sql updated to reflect actual two-table schema with
  managed_by column and migration comment.
- 11 new tests (130 total, all passing).

schema/coredns_mysql.sql

@@ -1,12 +1,33 @@
-CREATE TABLE IF NOT EXISTS `records` (
-  `id` int(11) NOT NULL AUTO_INCREMENT,
-  `zone` varchar(255) NOT NULL,
-  `name` varchar(255) NOT NULL,
-  `ttl` int(11) DEFAULT NULL,
-  `type` varchar(10) NOT NULL,
-  `data` text NOT NULL,
+-- DirectDNSOnly — CoreDNS MySQL schema
+-- Compatible with cybercinch/coredns_mysql_extend
+--
+-- managed_by values:
+--   'directadmin'  zone is managed via directdnsonly / DirectAdmin push
+--   'direct'       zone was created directly (not via DA)
+--   NULL           legacy row created before this column was added
+
+CREATE TABLE IF NOT EXISTS `zones` (
+  `id`          int(11)      NOT NULL AUTO_INCREMENT,
+  `zone_name`   varchar(255) NOT NULL,
+  `managed_by`  varchar(255) DEFAULT NULL,
   PRIMARY KEY (`id`),
-  KEY `idx_zone` (`zone`),
-  KEY `idx_name` (`name`),
-  KEY `idx_type` (`type`)
-) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
+  UNIQUE KEY `uq_zone_name` (`zone_name`)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
+
+CREATE TABLE IF NOT EXISTS `records` (
+  `id`        int(11)      NOT NULL AUTO_INCREMENT,
+  `zone_id`   int(11)      NOT NULL,
+  `hostname`  varchar(255) NOT NULL,
+  `type`      varchar(10)  NOT NULL,
+  `data`      text         NOT NULL,
+  `ttl`       int(11)      DEFAULT NULL,
+  `online`    tinyint(1)   NOT NULL DEFAULT 0,
+  PRIMARY KEY (`id`),
+  KEY `idx_zone_id`  (`zone_id`),
+  KEY `idx_hostname` (`hostname`),
+  CONSTRAINT `fk_records_zone` FOREIGN KEY (`zone_id`) REFERENCES `zones` (`id`)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
+
+-- Migration: add managed_by to an existing installation
+-- ALTER TABLE `zones` ADD COLUMN `managed_by` varchar(255) DEFAULT NULL;
+-- UPDATE `zones` SET `managed_by` = 'directadmin' WHERE `managed_by` IS NULL;