feat(search): add NL search error codes and i18n strings

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/main/java/org/raddatz/familienarchiv/exception/ErrorCode.java

@@ -135,6 +135,12 @@ public enum ErrorCode {
     /** The merge target is a descendant of the source tag. 400 */
     TAG_MERGE_INVALID_TARGET,
 
+    // --- NL Search ---
+    /** Ollama is unreachable or timed out. 503 */
+    SMART_SEARCH_UNAVAILABLE,
+    /** NL search rate limit exceeded (5 requests per user per minute). 429 */
+    SMART_SEARCH_RATE_LIMITED,
+
     // --- Generic ---
     /** Request validation failed (missing or malformed fields). 400 */
     VALIDATION_ERROR,