botapi: implement getChat

Adds the getChat method to the HTTP Bot API gateway. Numeric chat_id only (no
@username). Resolution goes through the shared peer resolvers:

- user: ByID; unknown -> CHAT_NOT_FOUND
- channel/supergroup: ResolveChannel, so a public one resolves even when the bot
  is not a member (projected as a preview); a private one the bot cannot access
  -> CHAT_NOT_FOUND, a banned bot likewise

The Chat projection returns id (bot-api encoded), type ("channel" for a
broadcast, "supergroup" for a megagroup, "private" for a user), title, username,
first/last name, description, is_forum, and the scam/fake/verified flags.

internal/rpc/botapi_gateway.go

@@ -33,6 +33,78 @@ func (r *Router) BotAPISelf(ctx context.Context, botID int64) (domain.User, erro
 	return u, nil
 }
 
+// BotAPIChat resolves a chat for the Bot API getChat method. chat_id is numeric
+// only (no @username). Public channels/supergroups resolve even when the bot is
+// not a member; a private chat the bot cannot access is CHAT_NOT_FOUND.
+func (r *Router) BotAPIChat(ctx context.Context, botID, chatID int64) (domain.BotAPIChat, error) {
+	if r == nil || botID == 0 {
+		return domain.BotAPIChat{}, errors.New("BOT_INVALID")
+	}
+	peer, ok := botAPIPeerFromChatID(chatID)
+	if !ok {
+		return domain.BotAPIChat{}, errors.New("CHAT_ID_INVALID")
+	}
+	switch peer.Type {
+	case domain.PeerTypeUser:
+		if r.deps.Users == nil {
+			return domain.BotAPIChat{}, errors.New("CHAT_NOT_FOUND")
+		}
+		u, found, err := r.deps.Users.ByID(ctx, botID, peer.ID)
+		if err != nil {
+			return domain.BotAPIChat{}, err
+		}
+		if !found {
+			return domain.BotAPIChat{}, errors.New("CHAT_NOT_FOUND")
+		}
+		return domain.BotAPIChat{
+			Peer:      peer,
+			Type:      "private",
+			FirstName: u.FirstName,
+			LastName:  u.LastName,
+			Username:  u.Username,
+			Verified:  u.Verified,
+			Scam:      u.Scam,
+			Fake:      u.Fake,
+		}, nil
+	case domain.PeerTypeChannel:
+		if r.deps.Channels == nil {
+			return domain.BotAPIChat{}, errors.New("CHAT_NOT_FOUND")
+		}
+		view, err := r.deps.Channels.ResolveChannel(ctx, botID, peer.ID)
+		if err != nil {
+			return domain.BotAPIChat{}, botAPIChatErr(err)
+		}
+		ch := view.Channel
+		typ := "supergroup"
+		if ch.Broadcast && !ch.Megagroup {
+			typ = "channel"
+		}
+		return domain.BotAPIChat{
+			Peer:        peer,
+			Type:        typ,
+			Title:       ch.Title,
+			Username:    ch.Username,
+			Description: ch.About,
+			IsForum:     ch.Forum,
+			Verified:    ch.Verified,
+			Scam:        ch.Scam,
+			Fake:        ch.Fake,
+		}, nil
+	}
+	return domain.BotAPIChat{}, errors.New("CHAT_ID_INVALID")
+}
+
+func botAPIChatErr(err error) error {
+	switch {
+	case errors.Is(err, domain.ErrChannelInvalid),
+		errors.Is(err, domain.ErrChannelPrivate),
+		errors.Is(err, domain.ErrChannelUserBanned):
+		return errors.New("CHAT_NOT_FOUND")
+	default:
+		return channelInvalidErr(err)
+	}
+}
+
 // BotAPIUpdates returns durable update_id based events projected for the HTTP
 // Bot API. New deployments use the dedicated Bot API queue; the legacy
 // user_update_events fallback is kept for tests that have not wired the queue.