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/styles.css

@@ -527,6 +527,31 @@ select {
   background: var(--accent-2);
 }
 
+/* ── Rail delete zone ───────────────────────────────────────────────────── */
+.rail-delete-zone {
+  margin-top: 24px;
+  padding-top: 20px;
+  border-top: 1px solid var(--line-soft);
+}
+.rail-delete-btn {
+  width: 100%;
+  padding: 8px 14px;
+  background: none;
+  border: 1px solid color-mix(in srgb, var(--accent) 45%, transparent);
+  color: var(--accent);
+  border-radius: var(--r);
+  cursor: pointer;
+  font-size: 12.5px;
+  font-weight: 600;
+  letter-spacing: 0.04em;
+  text-transform: uppercase;
+  transition: background 0.15s ease, border-color 0.15s ease;
+}
+.rail-delete-btn:hover {
+  background: color-mix(in srgb, var(--accent) 8%, transparent);
+  border-color: var(--accent);
+}
+
 /* ── Capture dialog ──────────────────────────────────────────────────────── */
 .capture-dialog {
   border: 1px solid var(--line);