chore: remediate pygienium audit findings

Dead code: 77 verified-unused exports, files (BackArrow, MenuBars,
cookies.ts, db/create.ts, schemas/comment.ts, security-headers.ts) and
12 unused dependencies removed
Comments: ~370 RESTATE comments stripped across 53 files; 2 verbose
blocks tightened; dead commented-out config removed
Complexity: bulkUpsert extracted into 11 per-entity helpers (CCN 156->~10);
login formHandler split into 3 submitters (CCN 63->~5); account page render
split into 8 section components (CCN 40); updatePost SQL builder rebuilt;
assert*Owned consolidated behind generic assertOwnedBy
Defensive guards: 4 redundant rethrow/nullish guards removed

src/server/cache.ts

@@ -100,7 +100,6 @@ export async function withCacheAndStale<T>(
   const now = Date.now();
   const entry = store.get(key) as CacheEntry<T> | undefined;
 
-  // Fresh hit
   if (entry && entry.expiresAt > now) return entry.data;
 
   try {
@@ -116,7 +115,6 @@ export async function withCacheAndStale<T>(
       console.error(`Error fetching data for cache key "${key}":`, error);
     }
 
-    // Stale fallback
     if (entry && entry.staleExpiresAt > now) {
       if (logErrors) console.log(`Serving stale data for cache key "${key}"`);
       return entry.data;