Add failure notification and resilience to backup services

- Add backup-b2-failed oneshot for OnFailure notification
- Add onFailure handler to both backup-b2 and backup-b2-check
- Add network-online.target dependency to backup-b2-check
- Add TimeoutStartSec (2h for backup, 1h for check)

Found via ops-review lenses.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

modules/backup-b2.nix

@@ -65,11 +65,26 @@ in
       };
     };
 
+    # Backup failure notification service
+    systemd.services.backup-b2-failed = {
+      description = "Handle backup failure notification";
+      serviceConfig = {
+        Type = "oneshot";
+        User = "root";
+      };
+      script = ''
+        echo "BACKUP FAILED at $(date)" | tee -a /var/log/backup-failures.log
+        echo "Check: journalctl -u backup-b2 -n 50"
+        # TODO: Add Matrix notification or healthchecks.io ping
+      '';
+    };
+
     # Backup service
     systemd.services.backup-b2 = {
       description = "Restic backup to Backblaze B2";
       after = [ "network-online.target" "postgresql.service" ];
       wants = [ "network-online.target" ];
+      onFailure = [ "backup-b2-failed.service" ];
       # Don't require postgres - backup should still run even if DB is down
       # (will just skip the dump files if they don't exist)
 
@@ -79,6 +94,8 @@ in
         # Low priority
         IOSchedulingClass = "idle";
         Nice = 19;
+        # Timeout after 2 hours to prevent hung backups
+        TimeoutStartSec = "2h";
       };
 
       path = [ pkgs.restic ];
@@ -126,12 +143,17 @@ in
     # Weekly integrity check service
     systemd.services.backup-b2-check = {
       description = "Verify B2 backup integrity";
+      after = [ "network-online.target" ];
+      wants = [ "network-online.target" ];
+      onFailure = [ "backup-b2-failed.service" ];
 
       serviceConfig = {
         Type = "oneshot";
         User = "root";
         IOSchedulingClass = "idle";
         Nice = 19;
+        # Timeout after 1 hour for integrity check
+        TimeoutStartSec = "1h";
       };
 
       path = [ pkgs.restic ];