Conform Mars appEvents-immutability fn to fleet §5.1 TG_OP-keyed amendment (6794d0e)

Venus landed the fleet §5.1 appEvents-immutability SSOT amendment (commit 6794d0e, pensanta-websites.git): fn_forbidAppEventsMutation bypass branch now keys return on TG_OP (DELETE->OLD, else->NEW) instead of unconditional RETURN OLD, which silently no-op'd any GUC-bypassed UPDATE (reported UPDATE N success, changed nothing). Task: VERIFY Mars's live immutability fn/trigger has the unconditional-RETURN-OLD shape (assume same fleet-standard shape but confirm, don't presume), then conform to the TG_OP-keyed form per 6794d0e. DDL -> db-mars owns, audit pre-apply review. NO live Mars impact: the only planned bypassed appEvents UPDATE is MARS-107 (categoryId backfill), still backlog/never-run, so no Mars data was silently dropped -> latent-correctness conform, not urgent. Venus one-time-backfill workaround (drop+recreate trigger byte-identical inside the migration txn, ACCESS EXCLUSIVE covers the immutability gap) available if MARS-107 runs before this conforms.