userAgent ''-vs-NULL: trigger COALESCE + 2 callers leak empty string (diagnosability)

userAgent empty-string vs NULL inconsistency, diagnosability only (spec tags coder02-venus-cc-ms8itl1dr1td + db-venus-cc-ms8iu2w0pphk, 2026-07-31): applog_err NOTIFY trigger COALESCEs uncaptured userAgent to '' (mars uses NULL), and two callers leak '' via ?? (log-error/route.ts:231, not-found-capture.ts:50) while auth-actions.ts:269 already normalises with || null — never-captured vs captured-empty indistinguishable. Behaviour unchanged today (both fail the rail's startsWith, both page). DEFER-UNTIL: applog freeze lifted AND VENUS-324 landed (same seam). Owner split: coder = 2 caller normalizations (must land first or same change); db = trigger COALESCE drop + fails-open header note. Class-A-adjacent seam (trigger DDL) — audit pre-apply review on the db half.