test(notify): add comprehensive tests for notification delivery

Add tests for sendNtfy, sendSlack, SendNotification, newRequest,
ntfyPriority, and slackColor covering:

- Correct ntfy headers (Title, Priority) and body content
- All priority-to-ntfy-priority mappings
- Correct Slack/Mattermost JSON payload structure and Content-Type
- All priority-to-color mappings
- HTTP error handling (4xx, 5xx status codes)
- Network transport failures
- Goroutine dispatch to all configured endpoints
- Individual endpoint dispatch (ntfy-only, slack-only, mattermost-only)
- No-op behavior when no webhooks are configured
- Error logging paths in SendNotification goroutines
- SlackPayload JSON marshaling and omitempty behavior
- HTTP request construction via newRequest
- Context propagation through newRequest

Uses httptest.Server for HTTP endpoint testing and a custom
failingTransport for network error simulation. Test helpers are
exported via export_test.go following standard Go patterns.

Coverage improved from 11.1% to 80.0% for the notify package.

internal/notify/export_test.go

@@ -0,0 +1,75 @@
+package notify
+
+import (
+	"context"
+	"io"
+	"log/slog"
+	"net/http"
+	"net/url"
+)
+
+// NtfyPriority exports ntfyPriority for testing.
+func NtfyPriority(priority string) string {
+	return ntfyPriority(priority)
+}
+
+// SlackColor exports slackColor for testing.
+func SlackColor(priority string) string {
+	return slackColor(priority)
+}
+
+// NewRequestForTest exports newRequest for testing.
+func NewRequestForTest(
+	ctx context.Context,
+	method string,
+	target *url.URL,
+	body io.Reader,
+) *http.Request {
+	return newRequest(ctx, method, target, body)
+}
+
+// NewTestService creates a Service suitable for unit testing.
+// It discards log output and uses the given transport.
+func NewTestService(transport http.RoundTripper) *Service {
+	return &Service{
+		log:       slog.New(slog.DiscardHandler),
+		transport: transport,
+	}
+}
+
+// SetNtfyURL sets the ntfy URL on a Service for testing.
+func (svc *Service) SetNtfyURL(u *url.URL) {
+	svc.ntfyURL = u
+}
+
+// SetSlackWebhookURL sets the Slack webhook URL on a
+// Service for testing.
+func (svc *Service) SetSlackWebhookURL(u *url.URL) {
+	svc.slackWebhookURL = u
+}
+
+// SetMattermostWebhookURL sets the Mattermost webhook URL on
+// a Service for testing.
+func (svc *Service) SetMattermostWebhookURL(u *url.URL) {
+	svc.mattermostWebhookURL = u
+}
+
+// SendNtfy exports sendNtfy for testing.
+func (svc *Service) SendNtfy(
+	ctx context.Context,
+	topicURL *url.URL,
+	title, message, priority string,
+) error {
+	return svc.sendNtfy(ctx, topicURL, title, message, priority)
+}
+
+// SendSlack exports sendSlack for testing.
+func (svc *Service) SendSlack(
+	ctx context.Context,
+	webhookURL *url.URL,
+	title, message, priority string,
+) error {
+	return svc.sendSlack(
+		ctx, webhookURL, title, message, priority,
+	)
+}