fix(review-2): critical — repair comment posting + close export data leak

CR1: the lightbox comment button POSTed to /upload/{id}/comment (singular);
     no such route exists, so every UI-submitted comment 404'd and was lost.
     Fixed to /comments (plural). The prior e2e passed because its seed helper
     POSTs the API directly — added comment-ui.spec.ts which drives the real
     component so this can't regress silently again.

CR2: export archives (Gallery.zip / Memories.zip / HTML viewer) were written
     under media_path, which is a public ServeDir — so GET /media/exports/
     Gallery.zip served the entire event (every photo, caption, comment,
     uploader name) to any anonymous visitor at a guessable URL, bypassing the
     ticket + release gate. Moved exports to a separate EXPORT_PATH (=/exports)
     on its own volume (Dockerfile chown, compose volume, gated handler reads
     the new path). export-leak.spec.ts asserts /media/exports/Gallery.zip → 404.

Riders (git can't split hunks): LightboxModal also gains H3 live-eviction on
comment-deleted/upload-deleted; config.rs also wires compression_concurrency
from boot config (medium).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.env.example

@@ -32,6 +32,9 @@ EVENT_SLUG=max-maria-2026
 
 # ── Storage ───────────────────────────────────────────────────────────────────
 MEDIA_PATH=/media
+# Export archives (Gallery.zip / Memories.zip). MUST be outside MEDIA_PATH —
+# /media is publicly served, so exports here would be downloadable without auth.
+EXPORT_PATH=/exports
 
 # ── Upload limits ─────────────────────────────────────────────────────────────
 DEFAULT_MAX_IMAGE_SIZE_MB=20