feat: implement entry deletion

- database: cascade_cached_bytes_after_subtree_delete — one-pass set-aware
  SQL that excludes the entire subtree simultaneously, avoiding sibling-blob
  cross-counting bug
- database: delete_entry — collects subtree IDs, runs set-aware cascade,
  NULLs archive_run_items.produced_entry_id (FK blocker), deletes children
  then root; ON DELETE CASCADE handles artifacts/tags/collections
- server: DELETE /api/archives/:archive_id/entries/:entry_uid route,
  wrapped in a transaction for atomicity
- frontend: deleteEntry API call, handleEntryDeleted callback in App.jsx
  (optimistic list removal + selection clear), Delete entry button in
  ContextRail with confirm guard
- tests: 4 database tests (unknown uid, subtree removal, run_item nulling,
  cached_bytes recalculation) + 3 route tests (204+gone, 404, 401)

frontend/src/App.jsx

@@ -212,6 +212,12 @@ export default function App() {
     )
   }, [])
 
+  const handleEntryDeleted = useCallback((entryUid) => {
+    setEntries(prev => prev.filter(e => e.entry_uid !== entryUid))
+    setSelectedEntry(prev => prev?.entry_uid === entryUid ? null : prev)
+    setSelectedEntryUid(prev => prev === entryUid ? null : prev)
+  }, [])
+
   const handleCaptureClick = useCallback(() => {
     setCaptureDialogOpen(true)
   }, [])
@@ -317,6 +323,7 @@ export default function App() {
             tagNodes={tagNodes}
             onTagsRefresh={handleTagsRefresh}
             onEntryTitleChange={handleEntryTitleChange}
+            onEntryDeleted={handleEntryDeleted}
             humanizeTags={humanizeTags}
           />
         </main>