Harden security: SSRF guard on all outbound fetches, auth fixes

- Route image-validation, Matrix image-download, and wayback/archive.today
  fetches through safehttp so feed-controlled URLs can't reach loopback/
  RFC1918/cloud-metadata IPs (incl. via redirects).
- Cap feed body size with safehttp.LimitedBody (16 MiB) to prevent OOM.
- Fail closed if matrix.pickle_key is unset/<16 chars; drop the hardcoded
  "pete_pickle_key" default that silently weakened E2EE-at-rest.
- Gate !post behind a matrix.admins allowlist; empty = disabled (channel
  rooms are public, so empty must not mean anyone).
- Reject /\host open-redirect bypass in post-login safeNext.
- Allowlist http(s) schemes for the Matrix link href; escape JSON embedded
  in inline <script> (prefs/sources blobs).

internal/web/auth.go

@@ -261,9 +261,12 @@ func (a *Authenticator) handleLogout(w http.ResponseWriter, r *http.Request) {
 	http.Redirect(w, r, "/", http.StatusFound)
 }
 
-// safeNext keeps post-login redirects on-site (no open redirects).
+// safeNext keeps post-login redirects on-site (no open redirects). It rejects
+// "//host" and "/\host" — browsers normalize the backslash to a slash, so the
+// latter would otherwise resolve to a protocol-relative off-site URL.
 func safeNext(next string) string {
-	if next == "" || !strings.HasPrefix(next, "/") || strings.HasPrefix(next, "//") {
+	if next == "" || !strings.HasPrefix(next, "/") ||
+		strings.HasPrefix(next, "//") || strings.HasPrefix(next, "/\\") {
 		return "/"
 	}
 	return next