Upgrade borgmatic 2.0.13 → 2.1.3 (#297)

## Summary
- Upgraded borgmatic from 2.0.13 to 2.1.3 on indri (via mise/pipx)
- Key changes: improved borg warning handling, memory/performance improvements, `source_directories_must_exist` now defaults to true (already set in our config)
- Verified: config validates, dry-run passed against both sifaka (local) and borgbase (offsite) repos

## Borg Warnings Investigation
The main concern was 2.1.0's change to treat borg warnings as errors. In 2.1.3 this was partially reverted — "file not found" warnings (exit code 107) are back to being warnings. Our config already sets `source_directories_must_exist: true`, and all four source directories were verified present on indri.

## Test plan
- [x] `borgmatic --version` confirms 2.1.3
- [x] `borgmatic config validate` passes
- [x] `borgmatic create --dry-run` succeeds against both repositories
- [x] All source directories verified present on indri
- [ ] Verify next scheduled backup (2:00 AM) completes successfully

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Reviewed-on: #297

docs/changelog.d/upgrade-borgmatic-2.1.3.infra.md

@@ -0,0 +1 @@
+Upgrade borgmatic from 2.0.13 to 2.1.3 on indri (improved borg warning handling, memory/performance improvements)

service-versions.yaml

@@ -283,9 +283,10 @@ services:
 
   - name: borgmatic
     type: ansible
-    last-reviewed: null
-    current-version: null
+    last-reviewed: 2026-03-16
+    current-version: "2.1.3"
     upstream-source: https://github.com/borgmatic-collective/borgmatic/releases
+    notes: Installed via mise (pipx), not managed by Ansible role
 
   - name: jellyfin
     type: ansible