fix(event-types): harden setup window + catalog validation (codex review)

Three review rounds on PR #801; fixes in response:

- isValidEventType: live catalog is authoritative when it has rows — a
  deleted or deactivated slug no longer validates via the legacy
  fallback (fallback now only serves an empty-catalog install)
- deleteEventType: refuse deleting the last (and last ACTIVE) type;
  updateEventType: refuse deactivating the last active type (unknown
  slugs are rejected since the validator change, so an empty active
  catalog would brick event creation)
- setup window fails closed: only an explicit stored `false` opens it
  (a portable-backup restore can leave the key absent) and a normal
  admin login durably closes it (abandoned-wizard case)
- reserved bootstrap keys (setup_wizard_completed, setup_token) are
  stripped from ALL generic settings upserts (/general, /security,
  /analytics, /seo) so the marker is genuinely one-way
- wizard step: deletes ordered so the catalog can never end up empty,
  and a genuinely failed system-type deletion reloads the list and
  stays on the step instead of advancing past the only window in which
  it can be retried
- CreateEventPage: snap the hardcoded initial 'wedding' selection to
  the first active type when the catalog no longer contains it
- v1 API: new GET /event-types (read scope) so token clients can
  discover valid slugs; OpenAPI enum replaced with the live-catalog
  description

frontend/src/i18n/locales/de.json

@@ -3622,7 +3622,8 @@
       "hint": "Das URL-Präfix erscheint in Galerie-Links (z.B. familie-mueller-2025-06-01). Symbole, Themes und Reihenfolge können Sie später unter Einstellungen → Veranstaltungsarten anpassen.",
       "atLeastOne": "Behalten Sie mindestens eine Veranstaltungsart — jede Galerie braucht eine.",
       "loadFailed": "Veranstaltungsarten konnten nicht geladen werden — Sie können sie später unter Einstellungen → Veranstaltungsarten anpassen.",
-      "saveFailed": "Einige Änderungen konnten nicht gespeichert werden — Sie können sie unter Einstellungen → Veranstaltungsarten abschließen."
+      "saveFailed": "Einige Änderungen konnten nicht gespeichert werden — Sie können sie unter Einstellungen → Veranstaltungsarten abschließen.",
+      "deleteFailed": "Eine Löschung ist fehlgeschlagen — die Liste wurde neu geladen. Mitgelieferte Arten können nur hier gelöscht werden; versuchen Sie es erneut oder fahren Sie mit ihnen fort."
     },
     "community": {
       "subtitle": "Alles bereit",

frontend/src/i18n/locales/en.json

@@ -3511,7 +3511,8 @@
       "hint": "The URL prefix appears in gallery links (e.g. family-smith-2025-06-01). Icons, themes and order can be tuned later in Settings → Event Types.",
       "atLeastOne": "Keep at least one event type — every gallery needs one.",
       "loadFailed": "Could not load the event types — you can adjust them later in Settings → Event Types.",
-      "saveFailed": "Some event type changes could not be saved — you can finish them in Settings → Event Types."
+      "saveFailed": "Some event type changes could not be saved — you can finish them in Settings → Event Types.",
+      "deleteFailed": "A deletion failed — the list has been reloaded. Built-in types can only be deleted here, so retry or continue with them kept."
     },
     "community": {
       "subtitle": "You're all set",