fix(favicon): short TTL for negative favicon cache entries

A failed favicon lookup was cached for a full week (same TTL as a
success), so a transient miss (e.g. the icon not yet indexed upstream)
blacklisted the domain for days. Cache negatives for 6 hours instead so
the next email retries.

Co-Authored-By: Claude Opus 4.7 <[email protected]>

src/config/constants.ts

@@ -31,6 +31,13 @@ export const STATS_KEY = "stats:counters";
 /** Default TTL for a cached per-domain favicon (seconds). */
 export const ICON_TTL_SECONDS = 7 * 24 * 60 * 60; // 1 week
 
+/**
+ * TTL for a *negative* favicon cache entry (seconds). Kept short so a transient
+ * miss (e.g. DuckDuckGo not having indexed the domain yet) self-heals within
+ * hours instead of blacklisting the domain for a full week.
+ */
+export const ICON_NEGATIVE_TTL_SECONDS = 6 * 60 * 60; // 6 hours
+
 /** Maximum accepted favicon size (bytes); larger responses are rejected. */
 export const MAX_ICON_BYTES = 100 * 1024; // 100 KB