feat(documents): expose content-index provenance on the document DTO

Surface the content-search index's source (vision vs local extraction) on
the list + detail DTO so the vault UI can tell an AI-read document from a
locally-indexed one. Threads the source through both read paths and the
serialisers, regenerates the OpenAPI contract, and adds a narrowing helper
for the free-String column.

docs/api/openapi.yaml

@@ -26193,6 +26193,15 @@ components:
             - attachment
         hasContentIndex:
           type: boolean
+        contentIndexSource:
+          anyOf:
+            - type: string
+              enum:
+                - vision
+                - text-ocr
+                - local-pdf
+                - local-ocr
+            - type: "null"
         createdAt:
           type: string
         updatedAt:
@@ -26214,6 +26223,7 @@ components:
         - conditionLinks
         - servingClass
         - hasContentIndex
+        - contentIndexSource
         - createdAt
         - updatedAt
       additionalProperties: false
@@ -26222,8 +26232,10 @@ components:
         (plaintext); `documentDate` is the user filing date; `reportDate` is the model-transcribed date (null until
         extraction runs). `servingClass` says how `/original` delivers the file — render inline (`inline`) or
         download-only (`attachment`); render/download by it, never by MIME guess. `hasContentIndex` is true when the
-        document has a content-search index (drives the re-index affordance). Treat an unknown `kind` value as OTHER
-        when decoding.
+        document has a content-search index (auto-indexed on upload); `contentIndexSource` says how that index was
+        produced — `vision` means an AI provider read the original, the other values are provider-free local
+        extractions, and it is null when `hasContentIndex` is false. Treat an unknown `kind` value as OTHER when
+        decoding.
     DocumentConditionLink:
       type: object
       properties:
@@ -26448,6 +26460,15 @@ components:
             - attachment
         hasContentIndex:
           type: boolean
+        contentIndexSource:
+          anyOf:
+            - type: string
+              enum:
+                - vision
+                - text-ocr
+                - local-pdf
+                - local-ocr
+            - type: "null"
         createdAt:
           type: string
         updatedAt:
@@ -26473,6 +26494,7 @@ components:
         - conditionLinks
         - servingClass
         - hasContentIndex
+        - contentIndexSource
         - createdAt
         - updatedAt
         - facts