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/002_snapshot_legacy_creator_names.sql

@@ -0,0 +1,42 @@
+-- Nachklang e.V. Calendar module — step 5 preparation, brought forward.
+-- Apply manually against the CALENDAR_DB database, after 001:
+--   mysql -h <DB_HOST> -u <DB_USER> -p <CALENDAR_DB> < 002_snapshot_legacy_creator_names.sql
+--
+-- Snapshots the creator's and last editor's *name* onto the event itself.
+--
+-- Why: the creator is only ever rendered as a name (nothing authorises on it),
+-- and today that name comes from joining the calendar's own `users` table.
+-- Step 5 drops that table, which would silently erase the authorship of every
+-- event created before the cutover. There is no account backfill to save them
+-- either - that was dropped deliberately, see docs/calendar-auth-migration.md.
+-- One text column per reference keeps the history at no ongoing cost.
+--
+-- These columns are an archive, not a source of truth. Nothing writes them
+-- after this backfill: events created from the cutover onwards carry an admin
+-- user id, whose name is resolved live so that renaming an account updates
+-- everywhere. The read path prefers the live admin name, falls back to this
+-- snapshot, and falls back again to the join until step 5 removes it.
+--
+-- The whole file is re-runnable: IF NOT EXISTS on the columns, and the backfill
+-- only touches rows with no snapshot yet. Step 4's migration re-runs the
+-- backfill, to catch anything created between this migration and the cutover.
+--
+-- No charset clause: unlike 001's id columns these hold display text that is
+-- only ever compared against other calendar data, so they inherit the tables'
+-- utf8mb4_general_ci like the columns they are copied from.
+
+ALTER TABLE `events`
+    ADD COLUMN IF NOT EXISTS `created_by_name` VARCHAR(255) NULL DEFAULT NULL AFTER `created_by_user_id`;
+
+ALTER TABLE `event_versions`
+    ADD COLUMN IF NOT EXISTS `version_created_by_name` VARCHAR(255) NULL DEFAULT NULL AFTER `version_created_by_user_id`;
+
+UPDATE `events` e
+    JOIN `users` u ON u.user_id = e.created_by_id
+    SET e.created_by_name = u.full_name
+    WHERE e.created_by_name IS NULL;
+
+UPDATE `event_versions` v
+    JOIN `users` u ON u.user_id = v.version_created_by_id
+    SET v.version_created_by_name = u.full_name
+    WHERE v.version_created_by_name IS NULL;