refactor(common): make init_observability sync with optional OTLP endpoint

Remove unnecessary async from init_observability -- body was fully sync.
Change otlp_endpoint from &str to Option<&str> -- when None, OTLP layer
is skipped (fmt-only mode). Update all 8 service callers.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

services/backtesting_service/src/main.rs

@@ -38,8 +38,8 @@ async fn main() -> Result<()> {
         .unwrap_or_else(|_| "http://localhost:4317".to_string());
     if let Err(e) = common::observability::init_observability(
         "backtesting_service",
-        &otlp_endpoint,
-    ).await {
+        Some(&otlp_endpoint),
+    ) {
         eprintln!("Failed to initialize observability: {}", e);
     }