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/strangler.ts

@@ -8,7 +8,7 @@
  * rollback — no redeploy, no code change. Setting `NEST_PREFIXES=` (empty) routes
  * everything back to the legacy app.
  */
-const DEFAULT_NEST_PREFIXES = ['/api/_nest'];
+const DEFAULT_NEST_PREFIXES = ['/api/_nest', '/api/weather'];
 
 export function getNestPrefixes(): string[] {
   const raw = process.env.NEST_PREFIXES;