Require NOCODB_BASE_ID: drop the baked-in fallback, fail with a clear message

- monitor.py: no default base id any more, and main() now reports whichever of
  NOCODB_TOKEN / NOCODB_BASE_ID is missing (writing the same text to
  /data/health.json) instead of dying inside bootstrap with a traceback
- docker-compose.yml: NOCODB_BASE_ID is a required variable now (production
  passes it in .env, so runtime behaviour is unchanged there)
- README: the contributing note matches reality again

Verified: python test_pagination.py passes, python -m py_compile clean, and a
run without the two variables prints "NOCODB_TOKEN, NOCODB_BASE_ID not set",
writes ok:false to health.json and exits 2.

docker-compose.yml

@@ -14,7 +14,7 @@ services:
     environment:
       NOCODB_URL: ${NOCODB_URL:-http://nocodb:10380}
       NOCODB_TOKEN: ${NOCODB_TOKEN}
-      NOCODB_BASE_ID: ${NOCODB_BASE_ID:-poqw1zjw3hnsk37}
+      NOCODB_BASE_ID: ${NOCODB_BASE_ID}
       TELEGRAM_BOT_TOKEN: ${TELEGRAM_BOT_TOKEN}
       TELEGRAM_CHAT_ID: ${TELEGRAM_CHAT_ID}
       TICK_SECONDS: ${TICK_SECONDS:-60}