fix(security): medium/low — event-lock, atomic config, a11y, diashow, hygiene

- social: likes/comments rejected on a closed event (e2e proven).
- admin: patch_config validates-then-writes atomically; char-based length.
- hashtag/comment models: atomic tag writes in edit + comment paths.
- Modal: inert the background (modal-inert action) for screen readers.
- sse.ts: idempotent visibilitychange listener (no duplicate reconnects).
- diashow: videos advance on `ended` (transitions + page wiring).
- docs/hygiene: README clone-case fix; removed stale committed .env.test and
  gitignored it; docker-compose.dev.yml tidy.

Left documented as accepted-risk per plan: PIN-persist-after-logout,
UUID-reachable hidden uploads, DECISION-media-auth.md.

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

backend/src/models/comment.rs

@@ -24,19 +24,24 @@ pub struct CommentDto {
 }
 
 impl Comment {
-    pub async fn create(
-        pool: &PgPool,
+    /// Takes any executor so the caller can insert the comment and link its
+    /// hashtags inside a single transaction.
+    pub async fn create<'e, E>(
+        executor: E,
         upload_id: Uuid,
         user_id: Uuid,
         body: &str,
-    ) -> Result<Self, sqlx::Error> {
+    ) -> Result<Self, sqlx::Error>
+    where
+        E: sqlx::PgExecutor<'e>,
+    {
         sqlx::query_as::<_, Self>(
             "INSERT INTO comment (upload_id, user_id, body) VALUES ($1, $2, $3) RETURNING *",
         )
         .bind(upload_id)
         .bind(user_id)
         .bind(body)
-        .fetch_one(pool)
+        .fetch_one(executor)
         .await
     }