HTTP API: register, list and remove webhooks, kept across restarts (closes #6)

POST, GET and DELETE under /api/v1/chats/{id}/webhooks, for the chats
that GET /api/v1/chats lists. The webhooks are kept in
$DATA_DIR/webhooks.json, mode 0600, which each change replaces whole
through a temporary file in the same directory and a rename. bot.Run
reads the file before it starts the chat client: absent means none, and
a file that cannot be read aborts startup. Reading a JSON request body
moved into decodeBody, which the messages endpoint now shares. Nothing
is posted to a webhook yet.

Model: opus-5-5

internal/config/config.go

@@ -64,8 +64,9 @@ var ErrInvalidConfig = errors.New("invalid configuration")
 // later, so there is exactly one moment at which configuration can be
 // wrong, and it is before anything starts.
 type Config struct {
-	// DataDir holds the SimpleX Chat database: the bot's profile, its
-	// address and its contacts. Losing it loses the address.
+	// DataDir holds the SimpleX Chat database, with the bot's profile,
+	// its address and its contacts, and the API's webhooks. Losing it
+	// loses the address.
 	DataDir string
 	Debug   bool