Adds services.arrInit.<name>.configXml for declaratively ensuring XML
elements exist in a Servarr config.xml before the service starts.
Generates a preStart hook on the main service that runs a Python helper
to patch or create config.xml. Undeclared elements are preserved;
declared elements are written with exact values.
Primary use case: preventing recurring Prowlarr 'not listening on port'
failures when config.xml loses the <Port> element — now guaranteed to
exist before Prowlarr starts.
Hardening:
- Atomic writes (tmp + rename): power loss cannot corrupt config.xml
- Malformed XML recovery: fresh <Config> root instead of blocking boot
- Secure default mode (0600) for new files containing ApiKey
- Preserves existing file mode on rewrite
- Assertion against duplicate serviceName targeting
Tests (10 subtests): creates-from-missing, patches-existing, preserves-
undeclared, corrects-tampered, idempotent, malformed-recovery,
ownership-preserved, not-world-readable.
