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/services/rate_limiter.rs

@@ -71,14 +71,21 @@ impl RateLimiter {
     }
 }
 
-/// Extract the client IP from X-Forwarded-For (Caddy sets this) or fall back
-/// to a provided socket address string.
+/// Extract the client IP from X-Forwarded-For or fall back to a provided socket
+/// address string.
+///
+/// We take the **right-most** entry, not the left-most. Caddy is the sole ingress
+/// and the app port is only `expose`d (never published), so the last hop Caddy
+/// appends is the real client. A client can prepend arbitrary spoofed values to
+/// the left of XFF to dodge throttles — those are ignored here. This assumes
+/// exactly one trusted proxy (Caddy); revisit if that changes.
 pub fn client_ip(headers: &axum::http::HeaderMap, fallback: &str) -> String {
     headers
         .get("x-forwarded-for")
         .and_then(|v| v.to_str().ok())
-        .and_then(|s| s.split(',').next())
+        .and_then(|s| s.rsplit(',').next())
         .map(|s| s.trim().to_owned())
+        .filter(|s| !s.is_empty())
         .unwrap_or_else(|| fallback.to_owned())
 }
 
@@ -134,9 +141,18 @@ mod tests {
     }
 
     #[test]
-    fn client_ip_prefers_first_forwarded_for_entry() {
+    fn client_ip_takes_rightmost_forwarded_for_entry() {
+        // The right-most entry is the hop our trusted proxy (Caddy) appended.
         let mut h = HeaderMap::new();
-        h.insert("x-forwarded-for", "203.0.113.7, 10.0.0.1".parse().unwrap());
+        h.insert("x-forwarded-for", "10.0.0.1, 203.0.113.7".parse().unwrap());
+        assert_eq!(client_ip(&h, "fallback"), "203.0.113.7");
+    }
+
+    #[test]
+    fn client_ip_ignores_spoofed_leftmost_entry() {
+        // A client prepending a fake IP to dodge throttles must not win.
+        let mut h = HeaderMap::new();
+        h.insert("x-forwarded-for", "1.2.3.4, 9.9.9.9, 203.0.113.7".parse().unwrap());
         assert_eq!(client_ip(&h, "fallback"), "203.0.113.7");
     }