HTTP API: register, list and remove webhooks, kept across restarts (closes #6)

POST, GET and DELETE under /api/v1/chats/{id}/webhooks, for the chats
that GET /api/v1/chats lists. The webhooks are kept in
$DATA_DIR/webhooks.json, mode 0600, which each change replaces whole
through a temporary file in the same directory and a rename. bot.Run
reads the file before it starts the chat client: absent means none, and
a file that cannot be read aborts startup. Reading a JSON request body
moved into decodeBody, which the messages endpoint now shares. Nothing
is posted to a webhook yet.

Model: opus-5-5

internal/api/api.go

@@ -1,5 +1,6 @@
 // Package api is the bot's HTTP API, through which another program
-// reads the bot's chats and sends messages in them.
+// reads the bot's chats, sends messages in them and registers webhooks
+// on them.
 //
 // Every request must carry the credential, as "Authorization: Bearer
 // {credential}"; with no credential configured, every request is
@@ -14,6 +15,8 @@ import (
 	"context"
 	"crypto/subtle"
 	"encoding/json"
+	"errors"
+	"io"
 	"log/slog"
 	"net/http"
 	"strconv"
@@ -75,6 +78,10 @@ type Params struct {
 	// Token is the credential every request must carry. Empty refuses
 	// every request.
 	Token string
+
+	// Webhooks holds the webhooks registered on the chats; ReadWebhooks
+	// makes it.
+	Webhooks *Webhooks
 }
 
 // New returns the API's server. The caller starts it with
@@ -84,7 +91,13 @@ func New(p Params) *http.Server {
 		p.Log.Warn("API_TOKEN_FILE is not set, so the API refuses every request")
 	}
 
-	h := &handlers{log: p.Log, client: p.Client, userID: p.UserID, token: p.Token}
+	h := &handlers{
+		log:      p.Log,
+		client:   p.Client,
+		userID:   p.UserID,
+		token:    p.Token,
+		webhooks: p.Webhooks,
+	}
 
 	router := chi.NewRouter()
 	router.Use(securityHeaders, h.authenticate,
@@ -99,6 +112,9 @@ func New(p Params) *http.Server {
 		r.Get("/chats", h.handleChats())
 		r.Get("/chats/{id}/messages", h.handleMessages())
 		r.Post("/chats/{id}/messages", h.handleSend())
+		r.Get("/chats/{id}/webhooks", h.handleWebhooks())
+		r.Post("/chats/{id}/webhooks", h.handleRegister())
+		r.Delete("/chats/{id}/webhooks/{webhook_id}", h.handleRemove())
 	})
 
 	return &http.Server{
@@ -119,10 +135,11 @@ func New(p Params) *http.Server {
 
 // handlers holds what the handlers share.
 type handlers struct {
-	log    *slog.Logger
-	client ChatClient
-	userID int64
-	token  string
+	log      *slog.Logger
+	client   ChatClient
+	userID   int64
+	token    string
+	webhooks *Webhooks
 }
 
 // authenticate lets a request through only if it carries the
@@ -165,6 +182,30 @@ func (h *handlers) respondError(w http.ResponseWriter, status int, sentence stri
 	}{sentence})
 }
 
+// decodeBody decodes the request's JSON body into v. If the body is too
+// large, or is not JSON of v's shape, it answers the request itself, 413
+// or 400 naming example as the shape wanted, and returns false.
+func (h *handlers) decodeBody(
+	w http.ResponseWriter, r *http.Request, v any, example string,
+) bool {
+	body, err := io.ReadAll(r.Body)
+
+	var tooLarge *http.MaxBytesError
+	if errors.As(err, &tooLarge) {
+		h.respondError(w, http.StatusRequestEntityTooLarge, "the body is too large")
+
+		return false
+	}
+
+	if err != nil || json.Unmarshal(body, v) != nil {
+		h.respondError(w, http.StatusBadRequest, "the body must be JSON such as "+example)
+
+		return false
+	}
+
+	return true
+}
+
 // securityHeaders go on every response. The API returns JSON to
 // programs, so a browser may not frame, sniff, cache or refer from it,
 // nor give it the camera, microphone or location, and must reach it