Rename express/ to backend/ and update references

Update paths in sync.sh, master/main.go, and CLAUDE.md to reflect
the directory rename.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/diachron/migrations/2026-01-24_01-roles-and-groups.sql

@@ -0,0 +1,20 @@
+CREATE TABLE roles (
+    id UUID PRIMARY KEY,
+    name TEXT UNIQUE NOT NULL,
+    description TEXT
+);
+
+CREATE TABLE groups (
+    id UUID PRIMARY KEY,
+    name TEXT NOT NULL,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+CREATE TABLE user_group_roles (
+    user_id UUID NOT NULL REFERENCES users(id),
+    group_id UUID NOT NULL REFERENCES groups(id),
+    role_id UUID NOT NULL REFERENCES roles(id),
+    granted_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    revoked_at TIMESTAMPTZ,
+    PRIMARY KEY (user_id, group_id, role_id)
+);