Refactor transit_api into a repo/routes module split

Split the 5,441-line index.js into an 86-line bootstrap, thin domain
route modules, shared lib/ helpers, and a repo/ layer that isolates the
DEV_MOCK branches (repo/*.mock.js). Repo methods return {statusCode, body};
routes are one-line wrappers. Pin search_path to "transit.bus" and
document the /bus/ prefix with legacy root aliases (Deprecation header).

Parity verified: 57/57 byte-identical on the stub differential harness
plus 24/24 live read-only responses vs the original against real
PostgreSQL. The only intentional behavior change: mock reorder/delete
bus-line endpoints no longer require a live DB (previously 500 in
DEV_MOCK=1 without PG). DEV_MOCK gate and node --check clean.

transit_api/lib/validate.js

@@ -0,0 +1,11 @@
+
+function isValidDateKey(value) {
+  if (!/^\d{4}-\d{2}-\d{2}$/.test(String(value || ""))) return false;
+  const date = new Date(`${value}T00:00:00Z`);
+  return (
+    !Number.isNaN(date.getTime()) && date.toISOString().slice(0, 10) === value
+  );
+}
+
+module.exports = { isValidDateKey };
+