feat: WeKan ticketing source

- wekan connector: board id as project key, cards assigned to the
  consultant's WeKan username (assignees with member fallback), archived
  cards skipped, client-side modifiedAt since-filtering, username/password
  login with token reuse (or pre-issued token), case-insensitive identity
- fake-WeKan unit tests: factory validation, test-connection, fetch
  filtering, key listing, token reuse
- admin customer wizard: WeKan option + credential fields
- OpenAPI enum + credential shapes, README identity mapping docs

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

internal/http/admin_customers.go

@@ -119,7 +119,7 @@ func (s *Server) handleAdminCreateCustomer(w http.ResponseWriter, r *http.Reques
 		return
 	}
 	if req.Type == nil || !req.Type.Valid() {
-		writeError(w, http.StatusBadRequest, "invalid_type", "ticketing type must be jira, azure_devops, youtrack or demo")
+		writeError(w, http.StatusBadRequest, "invalid_type", "ticketing type must be jira, azure_devops, youtrack, wekan or demo")
 		return
 	}
 	settings, err := s.store.GetSettings(r.Context())