feat: add OpenTelemetry trace spans to all handlers and pipeline functions

Add #[tracing::instrument(skip_all)] to 44 functions:
- 19 API handlers in mod.rs
- 3 chat handlers, 6 DAST handlers, 7 graph handlers
- 2 pipeline orchestrator functions
- 7 scanner functions (sbom, semgrep, gitleaks, cve, lint, patterns)

This generates trace spans for SigNoz visibility into request
latency, scan pipeline stages, and error tracking.

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

compliance-agent/src/pipeline/semgrep.rs

@@ -17,6 +17,7 @@ impl Scanner for SemgrepScanner {
         ScanType::Sast
     }
 
+    #[tracing::instrument(skip_all)]
     async fn scan(&self, repo_path: &Path, repo_id: &str) -> Result<ScanOutput, CoreError> {
         let output = tokio::process::Command::new("semgrep")
             .args(["--config=auto", "--json", "--quiet"])