Space watch URL fetches with FETCH_GAP_SECONDS (default 1s)

Same tick previously fired every due watch back-to-back (burst of N requests,
worst right after a restart when all watches are due at once). Now each watch
URL fetch is followed by a pause of FETCH_GAP_SECONDS (float, default 1, env
tunable; 0 disables) on both success and failure paths.

Also thread kw_fk_col into load_ignored_keywords() inside
send_pending_notifications() so the physical-FK lookup (445040a) actually takes
effect instead of silently falling back to the watch Link column.

DOCUMENTATION.md

@@ -57,6 +57,7 @@ Credentials are documented in `SECRETS.md` there.
 | `TELEGRAM_BOT_TOKEN` | *(secret)* | @carousellFoundBot |
 | `TELEGRAM_CHAT_ID` | `5648309582` | alert destination |
 | `TICK_SECONDS` | `60` | scheduler granularity |
+| `FETCH_GAP_SECONDS` | `1` | min pause (s) between watch URL fetches within one tick — anti-burst |
 | `HEALTH_STALE_SECONDS` | `600` | healthcheck staleness window |
 
 **Secrets = env vars (`.env`). Operational knobs = NocoDB Settings table.**