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/bot/run_test.go

@@ -182,6 +182,25 @@ func TestStopDuringRequest(t *testing.T) {
 	}
 }
 
+// TestUnreadableWebhooks: a webhooks file that cannot be read stops Run
+// before it starts the chat client.
+func TestUnreadableWebhooks(t *testing.T) {
+	// No chat client on PATH: starting one would fail with another error.
+	t.Setenv("PATH", t.TempDir())
+
+	cfg := &config.Config{DataDir: t.TempDir(), Port: freePort(t)}
+
+	err := os.WriteFile(filepath.Join(cfg.DataDir, "webhooks.json"), []byte("{"), 0o600)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	err = bot.Run(t.Context(), slog.New(slog.DiscardHandler), cfg, freePort(t))
+	if err == nil || !strings.Contains(err.Error(), "webhooks.json") {
+		t.Errorf("Run = %v, want an error naming webhooks.json", err)
+	}
+}
+
 // freePort returns a TCP port that nothing listens on at the moment.
 func freePort(t *testing.T) int {
 	t.Helper()