Add SearchXNG and SethSearch project context docs

api/README.md

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+# SethSearch API Layer
+
+## Live deployment
+
+- Host CT: 620 (`sethsearch-api`)
+- URL: `https://sethsearch.sethpc.xyz`
+- App: `/opt/sethsearch/sethsearch.py`
+- Config: `/opt/sethsearch/config.json`
+
+## Source groups
+
+- `source=general`: sites, gitea, wikijs, wordpress, freshrss, emby (strict)
+- `source=sethflix`: emby (liberal)
+
+## Weighting overview
+
+- Higher: sites, gitea, wikijs, wordpress, emby
+- Lower + strict: freshrss
+
+## Maintenance
+
+- Manual re-index: `POST /sync`
+- Health check: `GET /health`
+- Index summary: `GET /stats`