feat(city): add airports, ATC agent, luminous signs, and AI model fallback chain

- Add two airports (north/south) with full geometry: runways, taxiways,
  aprons, terminals, hangars, stands, and helipads; walkable airport
  grounds extend beyond city bounds.
- Introduce ATC AI agent ("Torre de Controle City10") using AI_ATC_MODEL
  to generate takeoff clearances; includes dispatch endpoint with
  30-second cooldown and deterministic fallback on failure.
- Implement AI model fallback chain: preferred model → AI_FALLBACK_MODEL
  before failing, applied in ChatController and PersonTalkController.
- Add luminous sign labels (letreiros) for plazas, beach, food, hospital,
  school, city hall, and airports with day/night glow.
- Fix city presence cache: compute is_me per viewer instead of baking
  into shared per-second cache to prevent cross-user flag leakage.
- Add tests: AirportTest, CityPresenceTest, SignLabelsTest, and
  PersonTalkTest fallback model retry scenario.

lang/pt_BR.json

@@ -219,6 +219,13 @@
     "city.beach_start": "Descansando na areia…",
     "city.beach_prompt": "Relaxar na praia",
     "city.beach_active": "Relaxando na praia",
+    "city.atc_missing": "Nenhuma torre de controle configurada.",
+    "city.sign_plaza": "Praça",
+    "city.sign_beach": "Praia",
+    "city.sign_food": "Comidas",
+    "city.sign_hospital": "Hospital",
+    "city.sign_school": "Escola",
+    "city.sign_cityhall": "Prefeitura",
     "city.exercise_need_equipment": "Aproxime-se de um equipamento da praça.",
     "city.exercise_need_awake": "Você precisa estar acordado para se exercitar.",
     "city.exercise_tired": "Você está cansado demais para se exercitar.",