Add offsite backup for immich photo library to BorgBase (#315)

## Summary

- Adds a second borgmatic config (`photos.yaml`) that backs up `/Volumes/photos` (sifaka SMB mount, ~128 GB) to a dedicated BorgBase repo (`immich-photos`), running daily at 4 AM
- Separate launchd agent (`mcquack.eblume.borgmatic-photos`) so photo backups run independently from the main backup
- Refactors `borgmatic_metrics` script to support multiple repos with a `repo` Prometheus label
- Updates Grafana "Borg Backups" dashboard with a `repo` template variable so you can filter/compare repos
- Docs updated: `backups.md`, `borgmatic.md`

## Prerequisites (manual)

- [x] Create `immich-photos` repo on BorgBase with same SSH key
- [ ] Upgrade BorgBase plan to Small ($24/yr) if currently on free tier (128 GB exceeds 10 GB limit)
- [ ] After deploy: `borg init` the new repo (borgmatic does this automatically on first run)

## Test plan

- [ ] Dry run: `mise run provision-indri -- --check --diff --tags borgmatic,borgmatic_metrics`
- [ ] Deploy borgmatic role and verify both configs deployed
- [ ] Run `borgmatic --config ~/.config/borgmatic/photos.yaml create --verbosity 1` manually for first backup (will take hours)
- [ ] Verify metrics script collects from both repos: `~/.local/bin/borgmatic-metrics && cat /opt/homebrew/var/node_exporter/textfile/borgmatic.prom`
- [ ] Sync grafana-config in ArgoCD and verify dashboard repo selector works

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

Reviewed-on: #315

docs/reference/services/borgmatic.md

@@ -15,9 +15,12 @@ Daily backup system using Borg backup, running on indri.
 | Property | Value |
 |----------|-------|
 | **Install** | mise (pipx) |
-| **Config** | `~/.config/borgmatic/config.yaml` |
-| **Schedule** | Daily at 2:00 AM |
-| **Repository** | `/Volumes/backups/borg/` on [[sifaka|Sifaka]] |
+| **Main config** | `~/.config/borgmatic/config.yaml` |
+| **Photos config** | `~/.config/borgmatic/photos.yaml` |
+| **Main schedule** | Daily at 2:00 AM |
+| **Photos schedule** | Daily at 4:00 AM |
+| **Main targets** | [[sifaka]] local + BorgBase offsite |
+| **Photos target** | BorgBase offsite only |
 
 ## What Gets Backed Up
 
@@ -35,6 +38,9 @@ Daily backup system using Borg backup, running on indri.
 **K8s SQLite databases (pre-backup dump via kubectl exec):**
 - [[mealie]] - Recipe manager (`/app/data/mealie.db`)
 
+**Immich photo library** (separate config, BorgBase offsite only):
+- `/Volumes/photos` (sifaka SMB mount, ~128 GB)
+
 **Not backed up (by design):**
 - ZIM archives (re-downloadable)
 - Prometheus metrics (ephemeral)