searxng: wire paid Kagi engine with deploy-time key rendering

The stock SearXNG image ships a native kagi engine; enabling it needs an
API key that must never enter git, and SearXNG does not expand env vars
in settings.yml (the live secret_key literally was '${SEARXNG_SECRET}'
until now — the app tolerated the placeholder string silently).

- settings.yml: kagi engine enabled (weight 2, kg shortcut, paging off so
  exactly one API credit per aggregated query, timeout 8.0 vs 5.0 scrapers
  so a slow paid query is not a wasted credit); api_key stays ${KAGI_API_KEY}.
- new scripts/searxng-render-settings.py: deploy-time renderer, allowlisted
  to KAGI_API_KEY/SEARXNG_SECRET/SEARXNG_BASE_URL, fails the deploy loudly
  on an unresolvable placeholder, idempotent on redeploys.
- deploy-remote.sh copy_searxng_config runs the renderer against
  /opt/searxng/.env after copying settings.yml into /opt/searxng/config.
- KAGI_API_KEY placeholder added to dankclaw/.env.example (real key staged
  into the host-side /opt/searxng/.env, mode 600, out of band).
- bats: engine wiring, renderer substitution, loud-failure, deployer seam.
- docs: SearXNG.md Kagi section + renderer law; Providers.md carve-out on
  the no-credentials policy (paid engine acceptable when bounded + ordered).

Verified live on dankclaw: direct API probe via the gluetun netns (200,
data.search[] shape), then an aggregated query returned 29/29 results
tagged kagi, zero unresponsive engines.

scripts/deploy-remote.sh

@@ -209,6 +209,11 @@ copy_searxng_config() {
     run_privileged mkdir -p /opt/searxng/config
     run_privileged cp shared/searxng/settings.yml /opt/searxng/config/settings.yml
     run_privileged cp shared/searxng/limiter.toml /opt/searxng/config/limiter.toml
+    # Bake ${KAGI_API_KEY}/${SEARXNG_SECRET}/${SEARXNG_BASE_URL} into the live
+    # copy from the host-side /opt/searxng/.env (SearXNG does not expand env
+    # vars in settings.yml). Fails the deploy if a placeholder is unresolvable.
+    run_privileged python3 scripts/searxng-render-settings.py \
+        /opt/searxng/.env /opt/searxng/config/settings.yml
 }
 
 copy_prometheus_config() {