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/config.rs

@@ -53,6 +53,11 @@ pub struct Config {
     pub llm_model: String,
     pub llm_temperature: f32,
     pub llm_timeout_seconds: u64,
+    /// System prompt sent to the analysis LLM. Defaults to
+    /// `crate::analyze::prompt::SYSTEM_PROMPT` if `LLM_SYSTEM_PROMPT` is unset
+    /// or empty. Runtime-configurable so admins can iterate and use the
+    /// "Neu analysieren" button to re-run cases.
+    pub llm_system_prompt: String,
     pub session_timeout_hours: u32,
     /// Whether to set the `Secure` flag on the session cookie. Default `true`.
     /// Set `COOKIE_SECURE=false` only for plain-HTTP local development —
@@ -87,6 +92,14 @@ impl Config {
             llm_model: optional_env("LLM_MODEL", ""),
             llm_temperature: optional_env_parsed("LLM_TEMPERATURE", 0.0),
             llm_timeout_seconds: optional_env_parsed("LLM_TIMEOUT_SECONDS", 180),
+            llm_system_prompt: {
+                let v = optional_env("LLM_SYSTEM_PROMPT", "");
+                if v.is_empty() {
+                    crate::analyze::prompt::SYSTEM_PROMPT.to_string()
+                } else {
+                    v
+                }
+            },
             session_timeout_hours: optional_env_parsed("SESSION_TIMEOUT_HOURS", 8),
             cookie_secure: optional_env_parsed("COOKIE_SECURE", true),
         }
@@ -135,6 +148,7 @@ impl Config {
             llm_model: String::new(),
             llm_temperature: 0.0,
             llm_timeout_seconds: 180,
+            llm_system_prompt: crate::analyze::prompt::SYSTEM_PROMPT.to_string(),
             session_timeout_hours: 8,
             cookie_secure: false,
         }