feat(alias): log the shape of a capture at debug level

The capture failures worth debugging are all about what Telegram did not
deliver: a reply stripped of its content, a caption where text was
expected, or a kind that falls through the switch. None of that is
visible from the user-facing refusal, which only says "unsupported".

One alias_capture line per /alias reports field names, lengths and
counts — never message text, so aliased messages cannot travel with the
logs.

internal/modules/alias/alias_debug.go

@@ -0,0 +1,87 @@
+package alias
+
+import (
+	"strings"
+
+	"github.com/go-telegram/bot/models"
+)
+
+// replyShape describes what /alias was handed, for the debug log.
+//
+// Enable with LOG_LEVEL=debug; silent otherwise. It exists because the failures
+// worth debugging here are all about what Telegram *did not* deliver: a reply
+// stripped of its content (another bot's message), a caption where text was
+// expected, or a kind that falls through capture's switch. Those are invisible
+// from the user-facing refusal, which only says the message was unsupported.
+//
+// Deliberately reports shape, never content. Lengths and field names are
+// enough to explain a capture failure, and message text in a log line would
+// mean every aliased message ending up in stdout and whatever ships it.
+func replyShape(replied *models.Message, entry Alias, ok bool) []any {
+	if replied == nil {
+		// The case that motivated this: Telegram can deliver /alias with no
+		// reply attached at all, which is indistinguishable from the caller
+		// forgetting to reply.
+		return []any{"reply", "absent"}
+	}
+
+	attrs := []any{
+		"reply", "present",
+		"reply_id", replied.ID,
+		"fields", strings.Join(populatedFields(replied), ","),
+		"text_len", len(replied.Text),
+		"caption_len", len(replied.Caption),
+		"entities", len(replied.Entities) + len(replied.CaptionEntities),
+		"captured", ok,
+	}
+	if ok {
+		attrs = append(attrs, "kind", entry.Kind)
+	}
+	if replied.From != nil {
+		attrs = append(attrs, "from_id", replied.From.ID, "from_bot", replied.From.IsBot)
+	} else {
+		// No sender at all — an anonymous channel post or a stripped reply.
+		attrs = append(attrs, "from", "absent")
+	}
+	if replied.SenderChat != nil {
+		attrs = append(attrs, "sender_chat", replied.SenderChat.ID)
+	}
+	return attrs
+}
+
+// populatedFields names the content fields the replied message actually has.
+//
+// Covers more than capture handles on purpose: the point is to show what
+// arrived, including kinds this module refuses, so "unsupported" can be told
+// apart from "empty".
+func populatedFields(m *models.Message) []string {
+	var out []string
+	add := func(present bool, name string) {
+		if present {
+			out = append(out, name)
+		}
+	}
+	add(m.Text != "", "text")
+	add(m.Caption != "", "caption")
+	add(m.Sticker != nil, "sticker")
+	add(len(m.Photo) > 0, "photo")
+	add(m.Animation != nil, "animation")
+	add(m.Video != nil, "video")
+	add(m.VideoNote != nil, "video_note")
+	add(m.Audio != nil, "audio")
+	add(m.Voice != nil, "voice")
+	add(m.Document != nil, "document")
+	add(m.Location != nil, "location")
+	add(m.Contact != nil, "contact")
+	add(m.Poll != nil, "poll")
+	add(m.Dice != nil, "dice")
+	add(m.Venue != nil, "venue")
+	add(m.Game != nil, "game")
+	add(m.ViaBot != nil, "via_bot")
+	add(m.ForwardOrigin != nil, "forward_origin")
+	if len(out) == 0 {
+		// The signature of a reply Telegram delivered but emptied.
+		out = append(out, "none")
+	}
+	return out
+}