Read calendar event creators from the admin module, and archive the old ones (#14)

Reviewed-on: #14
Co-authored-by: Patrick Müller <mail@pmueller.me>
Co-committed-by: Patrick Müller <mail@pmueller.me>

sql/calendar/001_add_admin_user_bridge.sql

@@ -0,0 +1,38 @@
+-- Nachklang e.V. Calendar module — step 1 of docs/calendar-auth-migration.md.
+-- Adds the bridging columns that let an event record who created it as an
+-- *admin* user id (VARCHAR(36)) alongside the legacy calendar users.user_id
+-- (INT). Apply manually against the CALENDAR_DB database:
+--   mysql -h <DB_HOST> -u <DB_USER> -p <CALENDAR_DB> < 001_add_admin_user_bridge.sql
+--
+-- Numbered 001 because this is the first migration this repo owns for the
+-- calendar schema: the tables themselves predate it and were provided by the
+-- repo owner (mirrored for dev in docker/init/01-calendar-schema-dev.sql).
+--
+-- Nothing reads these columns yet — step 3 introduces the dual-read. Adding
+-- them first means the backfill in step 2 has somewhere to write, and this
+-- migration can be applied to production on its own without any code change.
+--
+-- No foreign key, on purpose. The admin `user` table lives in a *different*
+-- database (nachklang_admin) behind a different connection pool, and a
+-- cross-schema FK would tie the two schemas' lifecycles together: you could no
+-- longer dump, restore or move one without the other. The reference is
+-- enforced in application code, which is also where the legacy/new fallback
+-- lives.
+--
+-- The collation is pinned to the admin database's (utf8mb4_unicode_ci) rather
+-- than inherited from the calendar tables' utf8mb4_general_ci. These columns
+-- hold ids that only ever compare against nachklang_admin.user.id, and a
+-- mismatched collation makes any such comparison fail at runtime with
+-- "Illegal mix of collations" instead of at review time.
+
+ALTER TABLE `events`
+    ADD COLUMN IF NOT EXISTS `created_by_user_id` VARCHAR(36)
+        CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci
+        NULL DEFAULT NULL AFTER `created_by_id`,
+    ADD KEY IF NOT EXISTS `events_created_by_user_idx` (`created_by_user_id`);
+
+ALTER TABLE `event_versions`
+    ADD COLUMN IF NOT EXISTS `version_created_by_user_id` VARCHAR(36)
+        CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci
+        NULL DEFAULT NULL AFTER `version_created_by_id`,
+    ADD KEY IF NOT EXISTS `event_versions_created_by_user_idx` (`version_created_by_user_id`);