Add LLM system prompt override

Allow overriding the LLM system prompt via the `LLM_SYSTEM_PROMPT`
environment variable. This provides flexibility for customizing LLM
behavior without code changes. A default prompt is used if the
environment variable is unset or empty.

server/src/analyze/worker.rs

@@ -80,7 +80,7 @@ async fn process(
     let document = match llm::chat_once(
         client,
         &settings,
-        prompt::SYSTEM_PROMPT,
+        &config.llm_system_prompt,
         &user_content,
         timeout,
     )