fix(security): high — live authz, XFF, SSE buffering, atomicity, pagination

H1: auth extractor re-reads the live user row — trusts the DB role (not the
    JWT claim) and rejects banned users mid-session. e2e proves a demoted
    host loses powers and a banned host is locked out and can't self-unban.
H2: client_ip takes the right-most XFF hop (the one Caddy appends); spoofed
    left-most entries are ignored, restoring IP-based throttles.
H3: Caddy excludes /api/v1/stream from `encode` so SSE isn't buffered.
H4: upload — quota increment + row insert + hashtag links now one txn.
H5: feed keyset pagination tiebroken on (created_at, id) + composite index
    (migration 010); feed_delta bounded with LIMIT.
H7: LightboxModal keys its comment load off upload.id, ending the
    SSE-driven refetch storm.
H8: CSP via SvelteKit kit.csp (svelte.config.js) hardens the localStorage
    token model against injection.

Migration 010 also adds the latent comment/comment_hashtag indexes.

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

backend/src/models/upload.rs

@@ -36,15 +36,20 @@ pub struct UploadDto {
 }
 
 impl Upload {
-    pub async fn create(
-        pool: &PgPool,
+    /// Takes any executor so the caller can run it inside a transaction (atomic
+    /// quota + insert) or standalone against the pool.
+    pub async fn create<'e, E>(
+        executor: E,
         event_id: Uuid,
         user_id: Uuid,
         original_path: &str,
         mime_type: &str,
         original_size_bytes: i64,
         caption: Option<&str>,
-    ) -> Result<Self, sqlx::Error> {
+    ) -> Result<Self, sqlx::Error>
+    where
+        E: sqlx::PgExecutor<'e>,
+    {
         sqlx::query_as::<_, Self>(
             "INSERT INTO upload (event_id, user_id, original_path, mime_type, original_size_bytes, caption)
              VALUES ($1, $2, $3, $4, $5, $6)
@@ -56,7 +61,7 @@ impl Upload {
         .bind(mime_type)
         .bind(original_size_bytes)
         .bind(caption)
-        .fetch_one(pool)
+        .fetch_one(executor)
         .await
     }
 
@@ -182,15 +187,18 @@ impl Upload {
         Ok(deleted)
     }
 
-    pub async fn update_caption(
-        pool: &PgPool,
+    pub async fn update_caption<'e, E>(
+        executor: E,
         id: Uuid,
         caption: Option<&str>,
-    ) -> Result<(), sqlx::Error> {
+    ) -> Result<(), sqlx::Error>
+    where
+        E: sqlx::PgExecutor<'e>,
+    {
         sqlx::query("UPDATE upload SET caption = $2 WHERE id = $1")
             .bind(id)
             .bind(caption)
-            .execute(pool)
+            .execute(executor)
             .await?;
         Ok(())
     }