activity: global feed, per-case log, sidebar section, today UX

- migration 009: target_type, target_id, target_path columns
- new Event fields: TargetType, TargetID, TargetPath
- ListActivityFeed (paginated global), ListActivityByNode (per-case)
- all Record() callsites pass target info
- frontend: Активность sidebar section with chronological feed
- per-case Активность tab with real data (was placeholder)
- today events: clickable, target-type badges, event counts

internal/core/nodes/types.go

@@ -47,8 +47,9 @@ var validSections = map[string]struct{}{
 
 // serviceSections are sidebar entries that are not stored as node sections.
 var serviceSections = map[string]struct{}{
-	"today": {},
-	"inbox": {},
+	"today":    {},
+	"inbox":    {},
+	"activity": {},
 }
 
 // IsValidType checks whether a type string is recognized.