fix: HTML export tojson filter + authenticated file download

- Enable minijinja 'json' feature so the tojson filter is available in
  the Memories.html template (was causing 'unknown filter' render error)
- Replace window.location.href downloads with fetch+blob so the
  Authorization header is sent (window.location.href caused 401)

DB: reset failed HTML export job to pending and cleared export_released_at
so the gallery can be re-released to trigger a fresh HTML export.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/Cargo.toml

@@ -29,7 +29,7 @@ sysinfo = "0.32"
 image = "0.25"
 oxipng = "9"
 async_zip = { version = "0.0.17", features = ["tokio", "deflate"] }
-minijinja = "2"
+minijinja = { version = "2", features = ["json"] }
 
 [profile.release]
 opt-level = 3