Use nfs-backup.toml and support completed status

Update agent to read nfs-backup.toml instead of legacy backup-status-*.toml
files. Add support for 'completed' status string used by backup script.

Changes:
- Read nfs-backup.toml from status directory
- Match 'completed' status as Status::Ok
- Simplify file scanning logic for single NFS backup file

Cargo.lock

@@ -279,7 +279,7 @@ checksum = "a1d728cc89cf3aee9ff92b05e62b19ee65a02b5702cff7d5a377e32c6ae29d8d"
 
 [[package]]
 name = "cm-dashboard"
-version = "0.1.267"
+version = "0.1.268"
 dependencies = [
  "anyhow",
  "chrono",
@@ -301,7 +301,7 @@ dependencies = [
 
 [[package]]
 name = "cm-dashboard-agent"
-version = "0.1.267"
+version = "0.1.268"
 dependencies = [
  "anyhow",
  "async-trait",
@@ -325,7 +325,7 @@ dependencies = [
 
 [[package]]
 name = "cm-dashboard-shared"
-version = "0.1.267"
+version = "0.1.268"
 dependencies = [
  "chrono",
  "serde",