docs(changelog): false_trigger_reason column under [Unreleased]

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YDXjZCr4RqT2U3QvMDhgzf

CHANGELOG.md

@@ -6,6 +6,17 @@ All notable changes to seismo-relay are documented here.
 
 ## [Unreleased]
 
+### Added
+- **`events.false_trigger_reason` — optional FT cause.** A nullable `TEXT`
+  column recording *why* an event is a false trigger (e.g. `"offset"`), as a
+  subtype of the FT flag: setting a reason via the sidecar review PATCH implies
+  `false_trigger=1`, and the reason is cleared whenever FT ends up 0
+  (confirm-real, clear-FT, `set_false_trigger(false)`). `propagate_review_to_twins`
+  carries the reason to the histogram/waveform twin alongside the flag.
+  Auto-migrated (`_SCHEMA` + `_migrate` ADD COLUMN — not the Migration-1
+  rebuild); exposed via `/db/events`. Terra-View surfaces it as a manual
+  "Flag as offset" action + an `FT · offset` badge.
+
 ---
 
 ## v0.28.0 — 2026-09-02