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/003_allow_null_legacy_creator.sql

@@ -0,0 +1,32 @@
+-- Nachklang e.V. Calendar module — step 4 of docs/calendar-auth-migration.md,
+-- the cutover. Apply manually against the CALENDAR_DB database, after 002,
+-- and BEFORE deploying the API build that goes with it:
+--   mysql -h <DB_HOST> -u <DB_USER> -p <CALENDAR_DB> < 003_allow_null_legacy_creator.sql
+--
+-- From the cutover on, an event's creator is an admin-module user id. There is
+-- no legacy calendar user id to write any more, and `events.created_by_id` is
+-- NOT NULL - so without this the very first event created after the deploy
+-- fails to insert. `event_versions.version_created_by_id` is already nullable.
+--
+-- The foreign key to `users` is kept: it permits NULL, so it costs nothing
+-- until step 5 drops the column and the table together.
+--
+-- Applying this early is harmless. Widening a column to accept NULL cannot
+-- break the running pre-cutover build, which always supplies a value, so this
+-- can go out ahead of the deploy rather than during it.
+
+ALTER TABLE `events`
+    MODIFY COLUMN `created_by_id` INT(11) NULL DEFAULT NULL;
+
+-- Re-run of 002's backfill, to catch anything created between the two
+-- migrations while the legacy path was still writing events. Idempotent by
+-- construction: it only touches rows that have no snapshot yet.
+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;