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/routes/health.js

@@ -0,0 +1,18 @@
+const express = require("express");
+const router = express.Router();
+const db = require("../db");
+const { log } = require("../lib/log");
+const { reportError } = require("../lib/report-error");
+
+router.get("/", async (_req, res) => {
+  try {
+    await db.query("SELECT 1");
+    res.json({ status: "ok", service: "transit_api", database: "ok" });
+  } catch (err) {
+    log("error", "GET /health error", { error: err.message, stack: err.stack });
+    reportError({ partofservice: "GET /health", error: err.stack || err.message });
+    res.status(503).json({ status: "unavailable", service: "transit_api" });
+  }
+});
+
+module.exports = router;