bugfix: security & correctness bundle (0.34.1)

Five fixes bundled into one release:

- preserve user-attached tags across crawler upserts
  (repo::crawler::sync_tags now scopes to added_by IS NULL; orphaned
  attachments from deleted users are reaped as crawler-owned)
- gate manga PATCH and cover endpoints on uploaded_by (require_can_edit
  in api::mangas; non-NULL uploaded_by must match the caller)
- equalise login response time across user-existence branches
  (run argon2 against a OnceLock-cached dummy hash on the no-user
  branch so timing doesn't leak username existence)
- crawler download defences (SSRF allowlist of host literals
  including IPv4-mapped IPv6 ranges, 32 MiB streamed size cap,
  reject non-whitelisted image types, three-way chapter-probe
  classifier replaces the binary #avatar_menu check)
- tighten validation and clean up dead unload path
  (attach_tag + create_token enforce 64-char caps; LocalStorage
  rejects NUL bytes explicitly; reader flushFinalProgress drops
  the always-405 sendBeacon path)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/src/repo/crawler.rs

@@ -274,7 +274,20 @@ async fn sync_tags(
     manga_id: Uuid,
     tags: &[String],
 ) -> sqlx::Result<()> {
-    sqlx::query("DELETE FROM manga_tags WHERE manga_id = $1")
+    // Only clear crawler-owned attachments (added_by IS NULL). User-
+    // attached tags are owned by the attaching user and must survive
+    // the recurring metadata pass — see manga_tags.added_by in
+    // migration 0009.
+    //
+    // Note on orphans: `manga_tags.added_by` is `ON DELETE SET NULL`,
+    // so an attachment whose user was deleted becomes
+    // indistinguishable from a crawler-owned row and is cleaned up
+    // here. That mirrors how `api::mangas::detach_tag` already treats
+    // orphans ("nobody owns it, refuse to let anyone but admin clear
+    // them") — the crawler now becomes the eventual reaper. Tracked
+    // by `sync_tags_garbage_collects_orphan_user_attachments` in
+    // backend/tests/crawler_sync.rs.
+    sqlx::query("DELETE FROM manga_tags WHERE manga_id = $1 AND added_by IS NULL")
         .bind(manga_id)
         .execute(&mut **tx)
         .await?;