refactor(api): consolidate health check endpoints to /health

Removes the redundant `/healthz` and `/api/v1/health` endpoints, leaving `/health` as the sole health check endpoint.

- Update router to return 404 Not Found for the removed endpoints
- Update admin log filtering to only ignore `/health`
- Remove health URL from API documentation data
- Update tests to verify `/health` returns 200 and others return 404
- Update README documentation to reflect the change

backend/templates/pages/api.html

@@ -14,7 +14,7 @@
         <h2>Endpoints</h2>
         <dl class="endpoint-list">
           <div><dt>Upload</dt><dd><code>POST /api/v1/upload</code></dd></div>
-          <div><dt>Health</dt><dd><code>GET /api/v1/health</code></dd></div>
+          <div><dt>Health</dt><dd><code>GET /health</code></dd></div>
           <div><dt>Request schema</dt><dd><a href="/api/v1/schemas/upload-request.json"><code>/api/v1/schemas/upload-request.json</code></a></dd></div>
           <div><dt>Response schema</dt><dd><a href="/api/v1/schemas/upload-response.json"><code>/api/v1/schemas/upload-response.json</code></a></dd></div>
         </dl>