perf(cache): sort-based eviction and consistent non-positive TTL handling

The count-based evictOldest used an O(n squared) selection sort with a
candidate cap over an unordered map range, so it could evict the wrong
entries. It now collects all entries and sorts by expiry, matching the
memory-based evictor.

A non-positive CACHE_TTL behaved differently per backend: the standard
memory cache panicked, Redis stored a permanent key, and the LRU cache
expired entries immediately. Each backend now clamps a non-positive TTL
to a shared default at its Set boundary. The nil-logger fallback in the
Redis wrapper now uses the constructor so it cannot panic.

cache/memory/lru_memory_cache.go

@@ -65,6 +65,14 @@ func (c *LRUMemoryCache) Set(key string, value []byte, ttl time.Duration) {
 	}
 
 	entrySize := int64(len(key) + len(finalValue) + 64) // 64 bytes overhead estimate
+
+	// A non-positive TTL is clamped to defaultTTL (defined in memory.go) so
+	// this backend does not stamp the entry as already-expired (immediate
+	// miss on every read). Keeps behavior identical to the standard memory
+	// backend and Redis, which clamp the same way at their own Set boundary.
+	if ttl <= 0 {
+		ttl = defaultTTL
+	}
 	expiresAt := time.Now().Add(ttl)
 
 	c.mu.Lock()