feat: build Go-based SearXNG-compatible search service

Implement an API-first Go rewrite with local engine adapters, upstream fallback, and Nix-based tooling so searches can run without matching the original UI while preserving response compatibility.

Made-with: Cursor

internal/contracts/types.go

@@ -0,0 +1,50 @@
+package contracts
+
+// OutputFormat matches SearXNG's `/search?format=...` values.
+type OutputFormat string
+
+const (
+	FormatHTML OutputFormat = "html" // accepted for compatibility (not yet implemented)
+	FormatJSON OutputFormat = "json"
+	FormatCSV  OutputFormat = "csv"
+	FormatRSS  OutputFormat = "rss"
+)
+
+type SearchRequest struct {
+	// Format is what the client requested via `format=...`.
+	Format OutputFormat
+
+	Query string
+
+	Pageno     int
+	Safesearch int
+	TimeRange  *string
+
+	TimeoutLimit *float64
+	Language     string
+
+	// Engines and categories are used for deciding which engines run locally vs are proxied.
+	// For now, engines can be supplied directly via the `engines` form parameter.
+	Engines    []string
+	Categories []string
+
+	// EngineData matches SearXNG's `engine_data-<engine>-<key>=<value>` parameters.
+	EngineData map[string]map[string]string
+
+	// AccessToken is an optional request token used to gate paid/limited engines.
+	// It is not part of the upstream JSON schema; it only influences local engines.
+	AccessToken string
+}
+
+// SearchResponse matches the JSON schema returned by SearXNG's `webutils.get_json_response()`.
+type SearchResponse struct {
+	Query                string         `json:"query"`
+	NumberOfResults     int            `json:"number_of_results"`
+	Results              []MainResult  `json:"results"`
+	Answers              []map[string]any     `json:"answers"`
+	Corrections          []string      `json:"corrections"`
+	Infoboxes            []map[string]any     `json:"infoboxes"`
+	Suggestions          []string      `json:"suggestions"`
+	UnresponsiveEngines [][2]string   `json:"unresponsive_engines"`
+}
+