fix(security): resolve Snyk/bandit code analysis findings

- config.py: move REPORT_OUTPUT_DIR from /tmp (world-writable) to /app/reports
  to prevent CWE-377 symlink attack vector (B108, only real security issue)
- main.py: log startup seed failures instead of silently swallowing them (B110)
- Add # nosec annotations to intentional try/except patterns that are by design:
  Jira integration errors, email failures, DetachedInstanceError, storage errors,
  and Jira session timeout (all B110/B112 false positives)
- Add # nosec B105 to false positives where bandit misidentifies config key
  names and masking strings as hardcoded passwords
- Add .bandit config to skip B311 in seed_demo.py (random used for fake
  demo data generation, not cryptographic purposes)

backend/app/routers/webhooks.py

@@ -36,7 +36,7 @@ def _mask_secret(wh) -> WebhookConfigOut:
     """Return a WebhookConfigOut with the secret masked."""
     out = WebhookConfigOut.model_validate(wh)
     if wh.secret:
-        out.secret = "***"
+        out.secret = "***"  # nosec B105
     else:
         out.secret = None
     return out