Add ClearCache to Authenticator

Used by Metacrypt when sealing to invalidate cached token validations.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

auth/cache.go

@@ -53,6 +53,13 @@ func (c *validationCache) get(tokenHash string) (*TokenInfo, bool) {
 	return entry.info, true
 }
 
+// clear removes all entries from the cache.
+func (c *validationCache) clear() {
+	c.mu.Lock()
+	c.entries = make(map[string]cacheEntry)
+	c.mu.Unlock()
+}
+
 // put stores TokenInfo in the cache with an expiration of now + TTL.
 func (c *validationCache) put(tokenHash string, info *TokenInfo) {
 	c.mu.Lock()