refactor(channels): consolidate media handling and improve resource cleanup

Extract common file download and audio detection logic to utils package,
implement consistent temp file cleanup with defer, add allowlist checks
before downloading attachments, and improve context management across
Discord, Slack, and Telegram channels. Replace logging with structured
logger and prevent context leaks in transcription and thinking animations.

pkg/channels/slack_test.go

@@ -172,22 +172,3 @@ func TestSlackChannelIsAllowed(t *testing.T) {
 		}
 	})
 }
-
-func TestTruncateStringSlack(t *testing.T) {
-	tests := []struct {
-		input  string
-		maxLen int
-		want   string
-	}{
-		{"hello", 10, "hello"},
-		{"hello world", 5, "hello"},
-		{"", 5, ""},
-	}
-
-	for _, tt := range tests {
-		got := truncateStringSlack(tt.input, tt.maxLen)
-		if got != tt.want {
-			t.Errorf("truncateStringSlack(%q, %d) = %q, want %q", tt.input, tt.maxLen, got, tt.want)
-		}
-	}
-}