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/domain/botapi_chat.go

@@ -0,0 +1,18 @@
+package domain
+
+// BotAPIChat is a peer resolved for the Bot API getChat method. The bot need
+// not be a member: a public channel or supergroup resolves (projected as a
+// preview), while a private chat the bot has no access to resolves to an error.
+type BotAPIChat struct {
+	Peer        Peer   // domain peer; the Bot API chat-id encoding is applied by the projection
+	Type        string // "private" | "group" | "supergroup" | "channel"
+	Title       string
+	Username    string
+	FirstName   string
+	LastName    string
+	Description string // channel/supergroup "about"
+	IsForum     bool
+	Verified    bool
+	Scam        bool
+	Fake        bool
+}