chore: harden system settings and unify prompt usage

Validate settings updates, use typed embedding backends, and route
ingestion through DB-stored prompts so admin edits take effect.

retrieval-pipeline/src/answer_retrieval_helper.rs

@@ -1,8 +1,5 @@
-use common::storage::types::system_prompts::DEFAULT_QUERY_SYSTEM_PROMPT;
 use serde_json::{json, Value};
 
-pub static QUERY_SYSTEM_PROMPT: &str = DEFAULT_QUERY_SYSTEM_PROMPT;
-
 pub fn get_query_response_schema() -> Value {
     json!({
        "type": "object",