Port Home Assistant backup script to Go (v3.1.1)

Rewrite home_assistant_backup.sh as a single Go binary that reads the
existing home_assistant_backup.conf unchanged.

- Create a backup over SSH, scp it locally, and prune old backups both
  on the HA host (when free space is low) and locally
- Send ntfy and Discord notifications at the same points as the script
- Drop the jq and yq dependencies: parse `ha backups --raw-json` and
  `ha backup new --raw-json` in Go, falling back to `slug:` line parsing
- Add timestamped progress output, a heartbeat while HA builds the
  backup, and live size/percent/speed reporting during the copy
- Build the Discord payload with json.Marshal and validate backup slugs
  before using them in remote commands
- Abort with a notification if the remote disk-space check fails
- Add -config and -version flags
- Build with CGO_ENABLED=0 for a static binary that runs on older glibc

V3/README.md

@@ -1,26 +1,22 @@
-# ha-backup
+# ha-backup v3.1.1
 
 Go port of `home_assistant_backup.sh`. Uses the same `home_assistant_backup.conf`.
 
 ## Build
-```
-CGO_ENABLED=0 go build -trimpath -ldflags="-s -w" -o ha-backup .
-```
+    CGO_ENABLED=0 go build -trimpath -ldflags="-s -w" -o ha-backup .
+
+Override the version at build time if needed:
+    CGO_ENABLED=0 go build -trimpath -ldflags="-s -w -X main.version=3.1.2" -o ha-backup .
+
+Check the version:
+    ./ha-backup -version
 
 Cross-compile examples:
-```
     GOOS=linux GOARCH=amd64 go build -o ha-backup-linux-amd64 .
     GOOS=linux GOARCH=arm64 go build -o ha-backup-linux-arm64 .   # Raspberry Pi / ARM NAS
-```
 
 ## Run
-#### looks for home_assistant_backup.conf next to the binary, then in cwd
-```
-./ha-backup
-```
-
-```  
-./ha-backup -config /path/to.conf
-```
+    ./ha-backup                       # looks for home_assistant_backup.conf next to the binary, then in cwd
+    ./ha-backup -config /path/to.conf
 
 Requires `ssh` and `scp` on the machine running it. `jq` and `yq` are no longer needed.