feat: append-only event log with projector skeleton

chat/db/migrations/0003_event_log.sql

@@ -0,0 +1,10 @@
+CREATE TABLE event_log (
+    id INTEGER PRIMARY KEY,
+    branch_id INTEGER NOT NULL DEFAULT 1,
+    ts TEXT NOT NULL DEFAULT (datetime('now')),
+    kind TEXT NOT NULL,
+    payload_json TEXT NOT NULL,
+    superseded_by INTEGER REFERENCES event_log(id),
+    hidden INTEGER NOT NULL DEFAULT 0
+);
+CREATE INDEX idx_event_log_branch_kind ON event_log(branch_id, kind);