feat(notifications): ntfy/Gotify push channels with live delivery (#69)

Per-user notification_channels (ntfy + Gotify): every
insertUserNotification now fans out best-effort to enabled channels —
deployment/backup/security/agent alerts reach the user's phone via the
ntfy app with zero extra infrastructure.

- POST/GET/DELETE /notifications/channels + POST .../:id/test which
  pushes a real probe so misconfiguration surfaces before alerts depend
  on it (502 DELIVERY_FAILED on endpoint errors).
- ntfy: POST to endpoint (topic in path), Title/Tags headers, optional
  Bearer token. Gotify: POST {endpoint}/message?token=...
- Delivery is best-effort by design — a dead relay never breaks the
  producing flow; one shared 5s-timeout client.
- CLI `notifications channels list|add|test|remove`, MCP tools,
  OpenAPI paths + NotificationChannel schema.
- Web settings "Push notifications" section and a Push channels card on
  the mobile notifications tab (add/test/remove).

Verified live: channel CRUD, test-ping hit a recording endpoint with
correct headers/body, and a real deployment fanned out "Deployment
succeeded" through the channel end-to-end.

Generated with [Devin](https://devin.ai)

Co-authored-by: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

app/backend/internal/api/notification_channels_test.go

@@ -0,0 +1,80 @@
+package api
+
+import (
+	"context"
+	"database/sql"
+	"encoding/json"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"containr/internal/database/sqlcdb"
+)
+
+func TestDeliverPushNtfy(t *testing.T) {
+	var gotAuth, gotTitle, gotTags, gotBody string
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		gotAuth = r.Header.Get("Authorization")
+		gotTitle = r.Header.Get("Title")
+		gotTags = r.Header.Get("Tags")
+		b, _ := io.ReadAll(r.Body)
+		gotBody = string(b)
+		w.WriteHeader(http.StatusOK)
+	}))
+	defer srv.Close()
+
+	ch := sqlcdb.NotificationChannel{
+		Kind:     "ntfy",
+		Endpoint: srv.URL + "/alerts",
+		Token:    sql.NullString{String: "tk_abc", Valid: true},
+	}
+	if err := deliverPush(context.Background(), ch, "deployment", "Deploy done", "svc is live"); err != nil {
+		t.Fatalf("deliverPush: %v", err)
+	}
+	if gotAuth != "Bearer tk_abc" {
+		t.Fatalf("auth header = %q", gotAuth)
+	}
+	if gotTitle != "Deploy done" || gotTags != "containr,deployment" || gotBody != "svc is live" {
+		t.Fatalf("title=%q tags=%q body=%q", gotTitle, gotTags, gotBody)
+	}
+}
+
+func TestDeliverPushGotify(t *testing.T) {
+	var gotPath, gotToken string
+	var payload map[string]interface{}
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		gotPath = r.URL.Path
+		gotToken = r.URL.Query().Get("token")
+		_ = json.NewDecoder(r.Body).Decode(&payload)
+		w.WriteHeader(http.StatusOK)
+	}))
+	defer srv.Close()
+
+	ch := sqlcdb.NotificationChannel{
+		Kind:     "gotify",
+		Endpoint: srv.URL,
+		Token:    sql.NullString{String: "apptoken", Valid: true},
+	}
+	if err := deliverPush(context.Background(), ch, "backup", "Backup done", "db backup ok"); err != nil {
+		t.Fatalf("deliverPush: %v", err)
+	}
+	if gotPath != "/message" || gotToken != "apptoken" {
+		t.Fatalf("path=%q token=%q", gotPath, gotToken)
+	}
+	if payload["title"] != "Backup done" || payload["message"] != "db backup ok" {
+		t.Fatalf("payload = %v", payload)
+	}
+}
+
+func TestDeliverPushFailure(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusInternalServerError)
+	}))
+	defer srv.Close()
+
+	ch := sqlcdb.NotificationChannel{Kind: "ntfy", Endpoint: srv.URL}
+	if err := deliverPush(context.Background(), ch, "k", "t", "b"); err == nil {
+		t.Fatal("expected error on 500")
+	}
+}