feat(backend): log file views

The activity.file_views table existed but nothing ever wrote to it. Add a
POST /files/{id}/views endpoint: FileRepo.RecordView inserts a history row,
FileService.RecordView enforces view ACL first. The file viewer fires it
(fire-and-forget) when a file is opened, including while paging prev/next.

Documented in openapi.yaml; covered by TestRecordFileView (204 on view,
repeatable, 404 for unknown file).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

openapi.yaml

@@ -407,6 +407,18 @@ paths:
         '404':
           $ref: '#/components/responses/NotFound'
 
+  /files/{file_id}/views:
+    post:
+      tags: [Files]
+      summary: Record that the current user viewed the file
+      parameters:
+        - $ref: '#/components/parameters/file_id'
+      responses:
+        '204':
+          description: View recorded
+        '404':
+          $ref: '#/components/responses/NotFound'
+
   /files/{file_id}/restore:
     post:
       tags: [Files]