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, whose WebSocket stays on 127.0.0.1 inside the container. Every request needs `Authorization: Bearer` with the credential from the file named by `API_TOKEN_FILE`, compared in constant time; with no credential configured every request is refused, `OPTIONS *` included. `GET /api/v1/chats` lists the bot's chats. Responses carry the security headers from the repository policies; bodies, requests and the server are time- and size-bounded. The chat client stops only after the API has finished its requests.

Disclosures: `contact_deleted` is an extra field; 404 and 405 answer in JSON; requests net/http cannot parse are refused by net/http without the security headers; three gosec findings are suppressed as false positives.

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})
+	}
+}