Add SearchXNG and SethSearch project context docs

context/ARCHITECTURE.md

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+# Architecture
+
+## Request flow
+
+1. User query enters SearXNG (`search.sethpc.xyz`).
+2. SearXNG calls `json_engine` endpoint at SethSearch API.
+3. SethSearch queries local SQLite FTS5 index and returns normalized results.
+4. SearXNG merges SethSearch with other engines and renders the result page.
+
+## Data plane
+
+- Index DB: `/opt/sethsearch/articles.db`
+- Tables:
+  - `documents` (canonical indexed records)
+  - `documents_fts` (FTS5 virtual table)
+- Source-level scoring and matching occur in SethSearch.
+
+## Source adapters
+
+- Caddy snapshot parser: domain discovery and tag generation.
+- Gitea adapter: public repo metadata via REST.
+- Wiki.js adapter: public crawl with fallback records.
+- WordPress adapter: public posts/pages via `/wp-json/wp/v2/...`.
+- Emby adapter: media index using server API token and deep links.
+- FreshRSS adapter: GReader API article ingest.
+
+## Reliability model
+
+- SethSearch syncs sources independently.
+- If one source fails, others continue and commit.
+- Service runs under systemd with restart policy.