Add migration to allow the extended zone kind in the database

The zones.kind CHECK constraint still only allowed 'forward', 'reverse4'
and 'reverse6', so creating an extended zone hit a constraint failure
and a 500. Rebuilds the zones table with the widened constraint while
preserving row ids so records.zone_id references stay valid.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01V8c76Xr2BycaHHu5N4buqi

internal/database/migrations/0003_zone_kind_extended.sql

@@ -0,0 +1,39 @@
+-- Adds the "extended" zone kind, which answers authoritatively only for the
+-- records it holds and falls through to recursion for everything else.
+--
+-- SQLite has no ALTER TABLE ... DROP/ADD CONSTRAINT, so the CHECK constraint
+-- on zones.kind is widened by rebuilding the table. The rebuilt table keeps
+-- the same name, column order and row ids, so records.zone_id references
+-- remain valid without touching the records table at all.
+
+CREATE TABLE zones_new (
+    id          INTEGER PRIMARY KEY AUTOINCREMENT,
+    name        TEXT    NOT NULL UNIQUE,
+    kind        TEXT    NOT NULL DEFAULT 'forward'
+                CHECK (kind IN ('forward', 'reverse4', 'reverse6', 'extended')),
+    description TEXT    NOT NULL DEFAULT '',
+    enabled     INTEGER NOT NULL DEFAULT 1,
+    default_ttl INTEGER NOT NULL DEFAULT 3600,
+    primary_ns  TEXT    NOT NULL,
+    admin_email TEXT    NOT NULL,
+    serial      INTEGER NOT NULL DEFAULT 1,
+    refresh     INTEGER NOT NULL DEFAULT 7200,
+    retry       INTEGER NOT NULL DEFAULT 3600,
+    expire      INTEGER NOT NULL DEFAULT 1209600,
+    minimum     INTEGER NOT NULL DEFAULT 3600,
+    auto_serial INTEGER NOT NULL DEFAULT 1,
+    created_at  INTEGER NOT NULL DEFAULT (unixepoch()),
+    updated_at  INTEGER NOT NULL DEFAULT (unixepoch())
+);
+
+INSERT INTO zones_new (id, name, kind, description, enabled, default_ttl, primary_ns,
+    admin_email, serial, refresh, retry, expire, minimum, auto_serial, created_at, updated_at)
+SELECT id, name, kind, description, enabled, default_ttl, primary_ns,
+    admin_email, serial, refresh, retry, expire, minimum, auto_serial, created_at, updated_at
+FROM zones;
+
+DROP TABLE zones;
+ALTER TABLE zones_new RENAME TO zones;
+
+CREATE INDEX idx_zones_enabled ON zones (enabled);
+CREATE INDEX idx_zones_kind ON zones (kind);