feat(fuchsia): implement backup strategy with explicit home paths

Add snapper and borgbackup for fuchsia home directory backups:

Snapper Configuration:
- Hourly snapshots of /home/sajenim
- Retention: 24 hourly, 7 daily, 4 weekly, 12 monthly
- Stored in nested .snapshots subvolume

Borgbackup Onsite:
- Backup to viridian over SSH (local network)
- Target: ssh://viridian/srv/borg-repo/fuchsia
- Hourly backups, unencrypted, deduplicated
- Same retention as snapper

Borgbackup Offsite:
- Backup to borgbase (internet)
- Target: li9kg944@li9kg944.repo.borgbase.com:repo
- Daily backups, encrypted (repokey-blake2), deduplicated
- Retention: 7 daily, 4 weekly, 12 monthly

Explicit Home Paths (valuable user data only):
- Documents, Pictures, Videos, Music, Downloads, Academics, Notes
- Dotfiles: .ssh, .gnupg

System Persist Data:
- SSH host keys, machine-id, nixos state
- Bluetooth, NetworkManager configurations

Intentionally Excluded:
- .config (managed declaratively via home-manager)
- .repositories (cloneable from GitHub)
- .cache and build artifacts

Treats viridian as central backup server, maintaining 3-2-1 strategy
(3 copies, 2 locations, 1 offsite).

chore(viridian): remove unused inputs parameter from borgbackup offsite

nixos/fuchsia/services/snapper/default.nix

@@ -0,0 +1,22 @@
+{...}: {
+  # Configure snapper for automated snapshots
+  # Snapshots stored in nested .snapshots subvolume within home directory
+  services.snapper = {
+    configs = {
+      # Home directory for user sajenim
+      home = {
+        SUBVOLUME = "/home/sajenim";
+        ALLOW_USERS = ["sajenim"];
+        TIMELINE_CREATE = true;
+        TIMELINE_CLEANUP = true;
+
+        # Tiered retention: 24h + 7d + 4w + 12m = ~1 year of snapshots
+        TIMELINE_LIMIT_HOURLY = 24;
+        TIMELINE_LIMIT_DAILY = 7;
+        TIMELINE_LIMIT_WEEKLY = 4;
+        TIMELINE_LIMIT_MONTHLY = 12;
+        TIMELINE_LIMIT_YEARLY = 0;
+      };
+    };
+  };
+}