fix(review): CSP nonce for FOUC script + feed_delta truncation signal

Two items surfaced by the branch re-review:

CSP regression (blocking): script-src 'self' blocked the template-authored
anti-FOUC theme script in app.html — SvelteKit's mode:'auto' only hashes
scripts it injects. Added nonce="%sveltekit.nonce%" so it's substituted per
request and included in the CSP (survives future edits, unlike a pinned hash).
Verified: emitted CSP nonce matches the script tag; no violation, no flash.

feed_delta silent truncation: the LIMIT 200 (added earlier to kill the
stale-`since` DoS) returned only the newest slice with no signal, so a client
that missed 200+ uploads during a reconnect could not tell it should full-
refresh — the older missed uploads were dropped and unrecoverable (the next
delta advances `since` past them). DeltaResponse now carries `truncated`; the
feed's feed-delta handler calls loadFeed(true) to resync from page 1 instead
of merging a partial slice.

Verified: cargo check clean, svelte-check 0 errors, production build green.

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

backend/src/handlers/feed.rs

@@ -172,6 +172,11 @@ pub struct DeltaQuery {
 pub struct DeltaResponse {
     pub uploads: Vec<FeedUpload>,
     pub deleted_ids: Vec<Uuid>,
+    /// True when the upload query hit `DELTA_LIMIT`: the response carries only the
+    /// newest slice of the gap, so the client must fall back to a full feed refresh
+    /// rather than merging (the older missed uploads are absent and unrecoverable
+    /// via a later delta, which advances `since` past them).
+    pub truncated: bool,
 }
 
 pub async fn feed_delta(
@@ -197,6 +202,10 @@ pub async fn feed_delta(
     .fetch_all(&state.pool)
     .await?;
 
+    // Hit the cap => this is only the newest slice of a larger gap. Signal the
+    // client to full-refresh instead of merging a partial delta.
+    let truncated = rows.len() as i64 >= DELTA_LIMIT;
+
     let deleted_ids: Vec<(Uuid,)> = sqlx::query_as(
         "SELECT id FROM upload
          WHERE event_id = $1 AND deleted_at IS NOT NULL AND deleted_at > $2",
@@ -229,6 +238,7 @@ pub async fn feed_delta(
     Ok(Json(DeltaResponse {
         uploads,
         deleted_ids: deleted_ids.into_iter().map(|r| r.0).collect(),
+        truncated,
     }))
 }
 

frontend/src/app.html

@@ -21,7 +21,10 @@
 			theme=dark don't flash a white screen. Mirrors the logic in
 			`src/lib/theme-store.ts`; kept in sync by hand (it's 6 lines).
 		-->
-		<script>
+		<!-- nonce is required: our CSP sets script-src 'self', and SvelteKit's
+		     mode:'auto' only hashes scripts *it* injects, not this template-authored
+		     one. %sveltekit.nonce% is substituted per request and added to the CSP. -->
+		<script nonce="%sveltekit.nonce%">
 			(function () {
 				try {
 					var pref = localStorage.getItem('eventsnap_theme') || 'system';

frontend/src/lib/types.ts

@@ -27,6 +27,9 @@ export interface FeedResponse {
 export interface DeltaResponse {
 	uploads: FeedUpload[];
 	deleted_ids: string[];
+	// True when the delta hit the backend cap and is only the newest slice of the
+	// gap — the client must full-refresh rather than merge (see feed-delta handler).
+	truncated: boolean;
 }
 
 // mirrors backend/src/handlers/feed.rs::HashtagCount

frontend/src/routes/feed/+page.svelte

@@ -206,6 +206,13 @@
 			onSseEvent('feed-delta', (data) => {
 				try {
 					const delta = JSON.parse(data) as DeltaResponse;
+					if (delta.truncated) {
+						// Missed more than the backend delta cap while backgrounded — the
+						// delta is only the newest slice, so merging would leave a silent gap
+						// of older-but-still-new uploads. Resync from page 1 instead.
+						void loadFeed(true);
+						return;
+					}
 					if (delta.uploads.length) {
 						const seen = new Set(uploads.map((u) => u.id));
 						const fresh = delta.uploads.filter((u) => !seen.has(u.id));