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/client.go

@@ -19,6 +19,7 @@ import (
 	"time"
 
 	"pete/internal/config"
+	"pete/internal/safehttp"
 
 	_ "go.mau.fi/util/dbutil/litestream" // registers "sqlite3-fk-wal" driver used by cryptohelper
 
@@ -430,14 +431,18 @@ func (c *Client) PostStory(channel string, story *PostableStory) (eventID id.Eve
 	return resp.EventID, imageSent, nil
 }
 
+// imageDownloadClient fetches feed-supplied image URLs for re-upload to Matrix.
+// It routes through safehttp so a hostile feed can't point ImageURL at internal
+// services (loopback / RFC1918 / cloud metadata) — including via redirects.
+var imageDownloadClient = safehttp.NewClient(15 * time.Second)
+
 // sendImage downloads an image, uploads it to Matrix, and sends it as m.image.
 func (c *Client) sendImage(ctx context.Context, roomID id.RoomID, imageURL string) error {
 	req, err := http.NewRequestWithContext(ctx, "GET", imageURL, nil)
 	if err != nil {
 		return fmt.Errorf("create image request: %w", err)
 	}
-	httpClient := &http.Client{Timeout: 15 * time.Second}
-	resp, err := httpClient.Do(req)
+	resp, err := imageDownloadClient.Do(req)
 	if err != nil {
 		return fmt.Errorf("download image: %w", err)
 	}