Fix summarizer worker infinite loop and map AI trigger errors to safe HTTP responses.

Paginate un-summarized tenders and mark rows missing notice/folder IDs as handled so the worker cannot spin forever. Return 400/404 for validation and not-found cases on AI summarize/analyze triggers instead of leaking internal errors as 500.

internal/tender/errors.go

@@ -0,0 +1,13 @@
+package tender
+
+import "errors"
+
+// Sentinel errors for AI trigger operations and HTTP mapping.
+var (
+	ErrEmptyTenderID                 = errors.New("tender: id is required")
+	ErrInvalidTenderID               = errors.New("tender: invalid id")
+	ErrTenderNotFound                = errors.New("tender: not found")
+	ErrTenderMissingNoticeID         = errors.New("tender: missing notice publication id")
+	ErrTenderMissingContractFolderID = errors.New("tender: missing contract folder id")
+	ErrAINotConfigured               = errors.New("tender: ai service not configured")
+)