Fix security scan issues and harden docker-compose

Hadolint fixes:
- Use printf instead of echo for escape sequences
- Add hadolint ignore for apt pin version (DL3008)

KICS fixes (docker-compose):
- Add security_opt: no-new-privileges to all services
- Add mem_limit and cpus to prevent resource exhaustion
- Add healthcheck to orchard-server in docker-compose.yml

Gitleaks:
- Add .gitleaksignore for false positive (s3_key attribute name)
- Remove allow_failure from secrets job (now blocking)

Also:
- Remove || echo fallback from python_tests (tests should fail pipeline)

.gitleaksignore

@@ -0,0 +1,6 @@
+# Gitleaks ignore file
+# https://github.com/gitleaks/gitleaks#gitleaksignore
+
+# False positive: s3_key is an attribute name, not a secret
+35fda65d381acc5ab59bc592ee3013f75906c197:backend/tests/unit/test_storage.py:generic-api-key:381
+08dce6cbb836b687002751fed4159bfc2da61f8b:backend/tests/unit/test_storage.py:generic-api-key:381