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/env.js

@@ -0,0 +1,8 @@
+// Environment/config shared by transit_api modules.
+const DEV_MOCK = process.env.DEV_MOCK === "1";
+const ERROR_LOG_API_URL =
+  process.env.ERROR_LOG_API_URL ||
+  "https://error.xtransit.testinstance.nl/error_log";
+
+module.exports = { DEV_MOCK, ERROR_LOG_API_URL };
+