feat: add source_url config option for footer source link

Thread source_url through: config.ServerConfig → Handler.sourceURL
→ PageData.SourceURL → template footer. Footer only shows Source
link when source_url is set.

config.example.toml

@@ -14,6 +14,11 @@ http_timeout = "10s"
 # Example: "https://search.example.com"
 base_url = ""
 
+# Link to the source code (shown in footer as "Source" link)
+# Defaults to the upstream kafka repo if not set.
+# Example: "https://git.example.com/my-kafka-fork"
+source_url = ""
+
 [upstream]
 # URL of an upstream metasearch instance for unported engines (env: UPSTREAM_SEARXNG_URL)
 # Leave empty to run without an upstream proxy.