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/database/sqlcdb/querier.go

@@ -44,6 +44,7 @@ type Querier interface {
 	DeleteBanner(ctx context.Context, id uuid.UUID) error
 	DeleteDatabaseServiceByID(ctx context.Context, id string) error
 	DeleteDatabaseServiceByIDAndUser(ctx context.Context, arg DeleteDatabaseServiceByIDAndUserParams) error
+	DeleteNotificationChannel(ctx context.Context, arg DeleteNotificationChannelParams) (int64, error)
 	DeleteOutboundWebhookByIDAndUser(ctx context.Context, arg DeleteOutboundWebhookByIDAndUserParams) error
 	DeleteProjectByID(ctx context.Context, projectID uuid.UUID) (int64, error)
 	DeleteProjectEnvironment(ctx context.Context, arg DeleteProjectEnvironmentParams) error
@@ -75,12 +76,14 @@ type Querier interface {
 	GetUserInviteByTokenHash(ctx context.Context, tokenHash string) (UserInvite, error)
 	GetUserTokenByHash(ctx context.Context, tokenHash string) (UserToken, error)
 	InsertAgentHeartbeat(ctx context.Context, arg InsertAgentHeartbeatParams) error
+	InsertNotificationChannel(ctx context.Context, arg InsertNotificationChannelParams) (NotificationChannel, error)
 	InsertProjectEnvironment(ctx context.Context, arg InsertProjectEnvironmentParams) error
 	ListActiveBanners(ctx context.Context) ([]Banner, error)
 	ListAgentAuthTokens(ctx context.Context) ([]AgentAuthToken, error)
 	ListAgentHeartbeatsSince(ctx context.Context, arg ListAgentHeartbeatsSinceParams) ([]AgentHeartbeat, error)
 	ListAgents(ctx context.Context) ([]NodeAgent, error)
 	ListAllDatabaseServices(ctx context.Context) ([]DatabaseService, error)
+	ListAllNotificationChannelsByUser(ctx context.Context, userID uuid.UUID) ([]NotificationChannel, error)
 	ListBackupTargetsByUser(ctx context.Context, userID string) ([]BackupTarget, error)
 	ListBanners(ctx context.Context) ([]Banner, error)
 	ListCommandsForAgent(ctx context.Context, nodeAgentID string) ([]AgentCommand, error)
@@ -90,6 +93,7 @@ type Querier interface {
 	ListDatabaseServicesByUser(ctx context.Context, userID string) ([]DatabaseService, error)
 	ListDueDatabaseBackups(ctx context.Context) ([]ListDueDatabaseBackupsRow, error)
 	ListEnabledOutboundWebhooks(ctx context.Context) ([]OutboundWebhook, error)
+	ListNotificationChannelsByUser(ctx context.Context, userID uuid.UUID) ([]NotificationChannel, error)
 	ListNotificationsByUser(ctx context.Context, arg ListNotificationsByUserParams) ([]Notification, error)
 	ListOutboundWebhooksByUser(ctx context.Context, userID uuid.UUID) ([]OutboundWebhook, error)
 	ListPendingCommands(ctx context.Context, nodeAgentID string) ([]AgentCommand, error)