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/matrix/post.go

@@ -3,9 +3,21 @@ package matrix
 import (
 	"fmt"
 	"html"
+	"net/url"
 	"strings"
 )
 
+// safeHref returns raw if it is an http(s) URL, else "". Keeps feed-supplied
+// links from injecting javascript:/data: schemes into the formatted_body href
+// (html.EscapeString blocks attribute breakout but not the scheme itself).
+func safeHref(raw string) string {
+	u, err := url.Parse(strings.TrimSpace(raw))
+	if err != nil || (u.Scheme != "http" && u.Scheme != "https") {
+		return ""
+	}
+	return raw
+}
+
 // PostableStory contains the data needed to format and send a story.
 type PostableStory struct {
 	ImageURL   string
@@ -30,11 +42,16 @@ func formatPost(s *PostableStory) (plain, htmlBody string) {
 
 	// HTML body
 	var htmlParts []string
-	htmlParts = append(htmlParts, fmt.Sprintf(
-		`<strong><a href="%s">%s</a></strong>`,
-		html.EscapeString(s.ArticleURL),
-		html.EscapeString(s.Headline),
-	))
+	if href := safeHref(s.ArticleURL); href != "" {
+		htmlParts = append(htmlParts, fmt.Sprintf(
+			`<strong><a href="%s">%s</a></strong>`,
+			html.EscapeString(href),
+			html.EscapeString(s.Headline),
+		))
+	} else {
+		// Non-http(s) link: render the headline without an anchor.
+		htmlParts = append(htmlParts, fmt.Sprintf(`<strong>%s</strong>`, html.EscapeString(s.Headline)))
+	}
 	if s.Lede != "" {
 		htmlParts = append(htmlParts, html.EscapeString(s.Lede))
 	}