feat: add POST support and compatibility params to /api/search

- Switch @router.get to @router.api_route with methods=[GET, POST]
- Add pageNumber alias for page (takes precedence when provided)
- Add numResults, format, imageProxy, safesearch params (accepted for
  compatibility with external clients)
- Update README.md, agents.md, FEATURES.md with param reference tables
- Add CHANGELOG 1.8.1 entry

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

CHANGELOG.md

@@ -1,6 +1,17 @@
 # Changelog
 
-## 1.8.0
+## 1.8.1
+
+### Added
+
+- **POST support on `/api/search`** — the search endpoint now accepts both GET and POST requests, enabling compatibility with clients that send search queries via POST
+- **Compatibility query parameters** on `/api/search`:
+  - `pageNumber` — alias for `page` (takes precedence when both are provided)
+  - `numResults` — requested result count hint (accepted and ignored; backend controls pagination)
+  - `format` — response format hint (e.g. `json`)
+  - `imageProxy` — client image-proxy preference flag (informational)
+  - `safesearch` — safe search level (`0` off, `1` moderate, `2` strict; accepted for compatibility)
+
 
 ### Added