feat(search): gator fields in oo_docs_text + field filters (#86)

- TextDoc carries gator keyword fields (doc_sha, number, seller, total, typ,
  issued_at, created_at, status, url); content is now omitempty so a partial
  doc can be written.
- oo_docs_text mapping extended with the keyword fields.
- ESTextIndex.UpsertFields: bulk update + doc_as_upsert, merging gator fields
  into an existing text doc without clobbering title/content.
- SearchQuery gains Seller/Number/Status term filters; SearchHit.Fields exposes
  the gator values. Pure builders unit-tested.

filestore_core.go

@@ -82,7 +82,8 @@ type FileStore interface {
 // SearchQuery narrows a Searcher request. InContent asks the backend to match
 // document bodies, not just titles. Substring switches title matching from the
 // analyzer's whole-token match to a case-insensitive "*term*" wildcard and ANDs
-// every whitespace-separated term (e.g. "rechnung 2025").
+// every whitespace-separated term (e.g. "rechnung 2025"). Seller/Number/Status
+// are exact filters over gator fields indexed into oo_docs_text (Option D).
 type SearchQuery struct {
 	Text       string
 	InContent  bool
@@ -90,15 +91,21 @@ type SearchQuery struct {
 	Extensions []string
 	Limit      int
 	Substring  bool
+	Seller     string
+	Number     string
+	Status     string
 }
 
 // SearchHit is one Searcher result: the matching entry plus backend-specific
-// ranking metadata.
+// ranking metadata. Fields carries backend-specific extra key/values (for
+// oo_docs_text: gator document fields such as rechnungsnummer, lieferant,
+// betrag, status).
 type SearchHit struct {
 	Entry
 	Score     float64
 	Highlight string
 	Path      []string
+	Fields    map[string]string
 }
 
 // Searcher is the optional content/name search surface. Only some backends