Add FK with ON DELETE CASCADE on occurrences.event_id

Was the last remaining gap in the occurrence-orphaning fix from earlier
this session: dependent: :destroy on Event#occurrences covers deletion
through ActiveRecord, this covers anything that bypasses it (raw SQL,
Model.delete). Confirmed zero existing orphans and zero NULL event_ids
before adding the constraint (event_id stays nullable - an occurrence
without an event is valid, one with a bogus event_id is not). Also adds
the index that should have existed on this column already - Postgres
needs one for the cascade to be anything other than a full table scan
per delete, and none existed before this. deferrable: :immediate added
so legitimate application code can still defer the check within a
transaction if ever needed - unrelated to, and not a fix for, the
fixture-loading issue below.

Applying this constraint broke fixture loading entirely (config fix:
81a07bf) and exposed a live bug in events#index (fix: 970f108), both
already committed separately - this migration file itself just never
got staged until now.

1 files changed, 6 insertions, 0 deletions

diff --git a/db/migrate/20260706014551_add_foreign_key_on_occurrences_event_id.rb b/db/migrate/20260706014551_add_foreign_key_on_occurrences_event_id.rb
new file mode 100644
index 0000000..a8df681
--- /dev/null
+++ b/db/migrate/20260706014551_add_foreign_key_on_occurrences_event_id.rb
@@ -0,0 +1,6 @@
+class AddForeignKeyOnOccurrencesEventId < ActiveRecord::Migration[8.1]
+  def change
+    add_index :occurrences, :event_id
+    add_foreign_key :occurrences, :events, on_delete: :cascade, deferrable: :immediate
+  end
+end