feat(weather): migrate /api/weather to the NestJS pilot module (L1) (#1053)

First strangler migration (L1): /api/weather is served by a NestJS module.

- @trek/shared/weather Zod contract; Nest controller byte-identical to the legacy Express route (paths, query params, status codes, { error } bodies, lang default, ApiError/500 passthrough). Service reuses getWeather/getDetailedWeather (+ shared cache; MCP tools unchanged).
- Strangler routes /api/weather to Nest by default; the legacy Express route + its migration-time parity test were decommissioned in this PR.
- Frontend (FE2): weatherApi typed against the @trek/shared WeatherResult contract.
- Harness: reusable Nest-vs-Express parity harness, e2e harness (temp SQLite + seed/cookie helpers, real JwtAuthGuard), src/nest coverage gate raised to >=80%, src/nest test guide.
- Verified end-to-end on a prod mirror (dev1): 401/400/200 via Nest with real Open-Meteo data, Express route gone.

server/src/nest/README.md

@@ -0,0 +1,58 @@
+# NestJS migration layer — module & test guide
+
+This folder holds the co-hosted NestJS app that incrementally strangles the legacy
+Express API (see the "Brownfield Rewrite" board). Until a prefix is migrated, the
+top-level dispatcher in `src/index.ts` routes it to the legacy app; migrated
+prefixes go to Nest. **Weather (`weather/`) is the reference implementation** — copy
+its shape when migrating a new domain.
+
+## Module layout (per domain)
+
+```
+shared/src/<domain>/<domain>.schema.ts(.spec.ts)   # Zod contract — single source of truth
+server/src/nest/<domain>/<domain>.service.ts        # business logic (ported 1:1 from the Express service)
+server/src/nest/<domain>/<domain>.controller.ts     # same routes/verbs/params/status codes as Express
+server/src/nest/<domain>/<domain>.module.ts         # registered in app.module.ts
+```
+
+Add the prefix to `DEFAULT_NEST_PREFIXES` in `strangler.ts` to route it to Nest
+(operators can override at runtime via the `NEST_PREFIXES` env var — instant
+rollback, no redeploy).
+
+## Parity is law
+
+A migrated route must be **byte-identical** for the client: same URL, method,
+query/body, HTTP status, `Set-Cookie`, and JSON body — including bespoke error
+strings. Where the legacy route returns a hand-written error (e.g. weather's
+`{ error: 'Latitude and longitude are required' }`), reproduce that exact body in
+the controller rather than relying on the generic `ZodValidationPipe` envelope.
+
+## How to write the tests
+
+Every module ships three kinds of tests; the coverage gate (`vitest.config.ts`,
+scoped to `src/nest/**`) requires ≥80%.
+
+1. **Service / controller unit spec** — `tests/unit/nest/<domain>.controller.test.ts`.
+   Instantiate the controller with a mocked service; assert status codes, the exact
+   `{ error }` bodies, and that inputs are forwarded correctly (defaults, coercion).
+   See `weather.controller.test.ts`.
+
+2. **Parity test** — `tests/parity/<domain>.parity.test.ts`. Mock the shared service
+   identically for both apps, then fire the same request at the Express route and the
+   Nest controller with the `expectParity()` harness (`tests/parity/parity.ts`) and
+   assert identical status + body. This is the gate before flipping the toggle.
+   See `weather.parity.test.ts`.
+
+3. **e2e** — `tests/e2e/<domain>.e2e.test.ts`. Boot the Nest module against a temp
+   in-memory SQLite db via the shared harness (`tests/e2e/harness.ts`:
+   `createTempDb`/`seedUser`/`sessionCookie`), exercising the **real** `JwtAuthGuard`
+   end-to-end (401 without cookie, 200 with a signed session). Mock external I/O
+   (HTTP/etc.). See `weather.e2e.test.ts`.
+
+## Definition of Done (per module)
+
+Contract in `@trek/shared` → service ported 1:1 → controller with identical routes →
+validation/error parity → unit + parity + e2e tests over the gate → prefix toggled to
+Nest → parity verified on the demo DB → **then** decommission the old Express
+route/service (separate step, after the toggle is confirmed in prod) → frontend points
+at the typed contract (Frontend Track).