notification: B3 — Content-Type header + URL trim

Fixes flagged in spec review of f0a323e: ntfy POSTs need explicit
Content-Type: text/plain (the spec calls for it; ntfy works without
but explicit beats inferred); trim trailing slashes from server URL
to avoid double-slash when operators paste 'https://ntfy.sh/'.

internal/notification/ntfy.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"io"
 	"net/http"
+	"strings"
 	"time"
 )
 
@@ -49,13 +50,15 @@ func (c *NtfyChannel) Kind() string { return "ntfy" }
 // with ntfy headers. Returns (statusCode, latency, err). 4xx/5xx
 // responses are returned as errors with the status code set.
 func (c *NtfyChannel) Send(ctx context.Context, p Payload) (int, time.Duration, error) {
-	url := c.cfg.ServerURL + "/" + c.cfg.Topic
+	server := strings.TrimRight(c.cfg.ServerURL, "/")
+	url := server + "/" + c.cfg.Topic
 
 	req, err := http.NewRequestWithContext(ctx, http.MethodPost, url, bytes.NewBufferString(p.Message))
 	if err != nil {
 		return 0, 0, fmt.Errorf("ntfy: build request: %w", err)
 	}
 
+	req.Header.Set("Content-Type", "text/plain")
 	req.Header.Set("Title", fmt.Sprintf("[%s] %s %s", p.Severity, p.HostName, p.Kind))
 	req.Header.Set("Tags", p.Severity+","+p.Kind)
 	req.Header.Set("Priority", priorityForSeverity(p.Severity, c.defaultPriority))