Add self-service event creation from nodes#show

nodes#show's events table now renders unconditionally (previously
hidden entirely for a zero-event node) and gains an "add event" link.
The suggested tag_list is derived from the page's own category tags
via NodesHelper::DEFAULT_EVENT_TAG_BY_PAGE_TAG (erfa-detail/
chaostreff-detail -> open-day) rather than hardcoded, and degrades to
a blank field for any node whose tags don't match - deliberately
universal, not chapter-specific, since Updates have historically
carried event dates the same way.

events#new surfaces *why* the tag was pre-filled via flash.now (not
flash - this is a same-request render, not a redirect), using an
explicit auto_tag_source param passed alongside tag_list rather than
having the controller re-derive the reason from node_id.

No destroy link added to nodes#show's events list - deliberate, per
existing subnav-semantics convention (destructive actions live on the
resource's own views). Covered directly by test.

Adds real coverage for EventsController, previously 100% commented-out
scaffold, plus unit tests for the new tag-mapping helper and the
weekday-abbreviation helpers from the prior commit.

1 files changed, 14 insertions, 0 deletions

diff --git a/app/helpers/nodes_helper.rb b/app/helpers/nodes_helper.rb
index 329bcc5..093bfc6 100644
--- a/app/helpers/nodes_helper.rb
+++ b/app/helpers/nodes_helper.rb
@@ -44,6 +44,20 @@ module NodesHelper
     ])
   end
 
+  DEFAULT_EVENT_TAG_BY_PAGE_TAG = {
+    'erfa-detail'       => 'open-day',
+    'chaostreff-detail' => 'open-day'
+  }.freeze
+
+  def default_event_tag_mapping(page)
+    page_tags = page.tag_list
+    DEFAULT_EVENT_TAG_BY_PAGE_TAG.find { |page_tag, _| page_tags.include?(page_tag) }
+  end
+
+  def default_event_tag_list(page)
+    default_event_tag_mapping(page)&.last
+  end
+
   def event_schedule_text(event)
     if event.rrule.present?
       recurrence = event.humanize_rrule(I18n.locale)