fix #63: require deletion token on DELETE /api/pastes/{id}

- DELETE now demands the create-time deletion token (Authorization
  header: Bearer/Token/bare, or ?token= query param), compared with
  the constant-time store.DeletionTokenEqual. 403 otherwise.
- Creator-browser deletes via the /mine button (matching vwr cookie,
  #37) remain allowed; other browsers and plain API clients get 403.
- Regression tests: no token, wrong token (header+query), correct
  token (header+query), creator-cookie path, token extraction.
- Adapted TestSoftDelete to pass the deletion token.
- docs/API.md delete section updated.
- Based on #58's SoftDelete (bool, error) signature.

internal/api/server.go

@@ -237,12 +237,12 @@ func (a *apiServer) handleDeletePaste(w http.ResponseWriter, r *http.Request) {
 		writeErr(w, 404, "paste not found")
 		return
 	}
-	// viewer-cookie delete enforcement (#37): only the browser that created
-	// the paste (matching vwr) may delete it via this endpoint. Requests with
-	// no client-sent vwr cookie (plain API clients) are unaffected.
-	vid := currentViewerID(r)
-	if vid != "" && viewerSentCookie(r) && row.ViewerID.Valid && row.ViewerID.String != "" && row.ViewerID.String != vid {
-		writeErr(w, 403, "not your paste")
+	// #63: deletion requires authorization. Either the deletion token issued
+	// at create time (Authorization header or ?token= query param, matching
+	// the create response's "deletion_token" field), or the creator browser
+	// itself (client-sent vwr cookie matching the paste's viewer, #37).
+	if !a.deletionAuthorized(r, row) {
+		writeErr(w, 403, "deletion token required")
 		return
 	}
 	if _, err := a.store.SoftDelete(row.ID); err != nil {
@@ -252,6 +252,37 @@ func (a *apiServer) handleDeletePaste(w http.ResponseWriter, r *http.Request) {
 	writeJSON(w, 200, map[string]string{"status": "soft-deleted"})
 }
 
+// deletionAuthorization extracts the deletion token from the request: the
+// Authorization header ("Bearer <t>", "Token <t>", or a bare token) or the
+// token query parameter. Returns "" when absent.
+func deletionAuthorization(r *http.Request) string {
+	if h := r.Header.Get("Authorization"); h != "" {
+		for _, prefix := range []string{"Bearer ", "Token "} {
+			if len(h) > len(prefix) && strings.EqualFold(h[:len(prefix)], prefix) {
+				return strings.TrimSpace(h[len(prefix):])
+			}
+		}
+		return strings.TrimSpace(h)
+	}
+	return r.URL.Query().Get("token")
+}
+
+// deletionAuthorized reports whether the request may soft-delete the paste:
+// a valid constant-time-matched deletion token, or the creator browser's
+// viewer cookie (#37). Plain API clients with no token get false.
+func (a *apiServer) deletionAuthorized(r *http.Request, row *store.PasteRow) bool {
+	if tok := deletionAuthorization(r); tok != "" {
+		return row.DeletionToken.Valid && row.DeletionToken.String != "" &&
+			store.DeletionTokenEqual(row.DeletionToken.String, tok)
+	}
+	// viewer-cookie delete enforcement (#37): only the browser that created
+	// the paste (matching vwr) may delete it via this endpoint. Requests with
+	// no client-sent vwr cookie (plain API clients) are unaffected.
+	vid := currentViewerID(r)
+	return vid != "" && viewerSentCookie(r) && row.ViewerID.Valid &&
+		row.ViewerID.String != "" && row.ViewerID.String == vid
+}
+
 // handleListMine serves /api/mine: pastes created from this browser (#37).
 func (a *apiServer) handleListMine(w http.ResponseWriter, r *http.Request) {
 	vid := currentViewerID(r)