Add SSE updates

backend/src/life_towers/migrations/002_revision.sql

@@ -0,0 +1,7 @@
+-- Per-user revision counter for multi-client sync.
+-- Bumped inside the same transaction as every PUT /data so clients can:
+--   * detect another client's write (cheap compare, no full-tree diff), and
+--   * guard their own writes with compare-and-swap (stale If-Match -> 409).
+-- Existing users start at 0; their next confirmed GET seeds the client base.
+
+ALTER TABLE users ADD COLUMN revision INTEGER NOT NULL DEFAULT 0;