fix: zero compiler warnings + readiness endpoint

- Fix all 106 warnings: unused imports, dead struct fields (_prefix),
  dead_code annotations on bin-only items in ingest_worker.rs
- Add GET /ready endpoint (readiness probe, checks DB)
- Keep GET /health lightweight (liveness, no DB check)
- Add ServiceMonitor for Prometheus scraping
- Add tracing-subscriber json feature for structured logs
- 760 tests passing, 0 warnings, 0 errors

crates/mem-cli/src/agent/client_sdk.rs

@@ -74,7 +74,7 @@ impl ClientResponse {
 /// Synthesis client SDK with JWT auth support + pod-aware routing
 pub struct SynthesisClient {
     base_url: String,       // Resolved URL (internal or external)
-    external_url: String,   // Fallback external URL
+    _external_url: String,   // Fallback external URL
     jwt_token: String,      // JWT Bearer token for all requests
     timeout_secs: u32,
     is_pod: bool,           // Running inside k8s pod?
@@ -102,7 +102,7 @@ impl SynthesisClient {
 
         SynthesisClient {
             base_url,
-            external_url,
+            _external_url: external_url,
             jwt_token,
             timeout_secs,
             is_pod,
@@ -369,7 +369,7 @@ mod tests {
             SynthesisClient::new("https://api.riotpiao.com".to_string(), "test-jwt-placeholder".to_string());
         
         // Verify ConfigMap env vars respected
-        assert!(!client.external_url.is_empty());
+        assert!(!client._external_url.is_empty());
         assert_eq!(client.timeout_secs, 45);
     }
 
@@ -459,7 +459,7 @@ mod tests {
     fn test_external_fallback_url() {
         let client =
             SynthesisClient::new("https://api.riotpiao.com".to_string(), "jwt".to_string());
-        assert_eq!(client.external_url, "https://api.riotpiao.com");
+        assert_eq!(client._external_url, "https://api.riotpiao.com");
     }
 
     #[test]