HTTP API: server, credential and the list of chats (closes #4)

The bot now serves an HTTP API on PORT (default 8080) beside the chat
client. Every request needs the credential read at startup from the
file named by API_TOKEN_FILE, sent as a bearer token; without one,
every request is refused. Responses carry the security headers, bodies
are capped at 64 KiB and each request's work at 10 seconds.

GET /api/v1/chats lists the bot's contacts from the chat client's
/_contacts command, ordered by id, and marks the contacts who deleted
their chat with the bot, which the chat client keeps listing. bot.Run
starts the API after set-up and stops it within 5 seconds; a listener
failure ends the bot as a chat client failure does.

Model: opus-5-5

internal/api/chats.go

@@ -0,0 +1,46 @@
+package api
+
+import (
+	"cmp"
+	"net/http"
+	"slices"
+)
+
+// handleChats lists the bot's chats, ordered by id. The bot talks to
+// people only one to one, so its chats are its contacts, and a chat's
+// id is its contact's.
+func (h *handlers) handleChats() http.HandlerFunc {
+	type chat struct {
+		ID             int64  `json:"id"`
+		DisplayName    string `json:"display_name"`
+		ContactDeleted bool   `json:"contact_deleted"`
+	}
+
+	type response struct {
+		Chats []chat `json:"chats"`
+	}
+
+	return func(w http.ResponseWriter, r *http.Request) {
+		contacts, err := h.client.Contacts(r.Context(), h.userID)
+		if err != nil {
+			h.log.Error("listing the chats", "error", err)
+			h.respondError(w, http.StatusInternalServerError,
+				"the chats could not be read")
+
+			return
+		}
+
+		chats := make([]chat, 0, len(contacts))
+		for _, c := range contacts {
+			chats = append(chats, chat{
+				ID:             c.ContactID,
+				DisplayName:    c.Profile.DisplayName,
+				ContactDeleted: c.Deleted(),
+			})
+		}
+
+		slices.SortFunc(chats, func(a, b chat) int { return cmp.Compare(a.ID, b.ID) })
+
+		h.respond(w, http.StatusOK, response{Chats: chats})
+	}
+}