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.

serverAppConfigs-docs/Dankclaw/SearXNG Providers.md

@@ -363,8 +363,15 @@ misleading).
    disable pagination for rate-sensitive engines.
 3. **Promote by evidence** — raise `weight` only for engines that return useful
    results consistently; weight is not a health check.
-4. **Avoid credentials** — do not add API-backed engines unless the provider is
-   genuinely free, anonymous, and compatible with our privacy requirements.
+4. **Avoid credentials, with one carve-out** — do not add API-backed engines
+   unless the provider is genuinely free, anonymous, and compatible with our
+   privacy requirements. Carve-out (2026-09-26): a paid, user-ordered API
+   engine (Kagi) is acceptable when the blast radius is bounded — the
+   `kagi` engine runs with `paging: false` and weight 2, so it spends exactly
+   one API credit per aggregated query, and its key never enters git
+   (deploy-time rendering from `/opt/searxng/.env`; see [[SearXNG]]). It is
+   also immune to the shared-VPN-exit-IP blocking this page is about: it
+   authenticates, so reputation of the exit IP is irrelevant.
 5. **Verify first** — `/json` + `?engines=google` and the searx-checker to see
    exactly which engines are down and why. Confirm the running SearXNG version
    (check whether `parsers`/`rate_limit` exist in the pinned build).