Unify duplicate TelegramNotifier and DiscordNotifier interfaces

[msoedov]

internal/handlers/api.go:23-31 -- TelegramNotifier and DiscordNotifier define identical method sets:

• SendWorkBlockApproval(blockID, title, goal, transition string) error
• SendMessage(text string) error

Both telegram.Bot and discord.Notifier already satisfy either interface interchangeably.

Proposal:

• Replace both with a single Notifier interface in api.go
• Change the API struct to hold notifiers []Notifier instead of separate telegram and discord fields
• Update cmd/secondorder/main.go initialization accordingly

Files affected: internal/handlers/api.go, cmd/secondorder/main.go, internal/telegram/bot.go, internal/discord/webhook.go