feat: add channels directory with 44 channel adapter definitions (#21)

* feat: add channels directory with 44 channel adapter definitions

Add TOML definitions for all 44 channel adapters supported by LibreFang,
matching the adapters in librefang-channels crate source code.

Each channel file includes: id, name, description, category, icon,
protocol, and metadata (url, docs).

Categories: messaging, social, enterprise, developer, iot, email.

Also adds channels template at templates/channel.toml.

* fix: format templates/channel.toml with taplo

channels/ntfy.toml

@@ -0,0 +1,10 @@
+id = "ntfy"
+name = "ntfy"
+description = "ntfy adapter for sending push notifications via the ntfy pub-sub service"
+category = "iot"
+icon = "🔔"
+protocol = "rest-api"
+
+[metadata]
+url = "https://ntfy.sh"
+docs = "https://docs.ntfy.sh"