M2 drag-reorder: notes.position + reorder API + native DnD

- Migration 0008: notes.position (int). Board orders pinned -> position ->
  updated_at; new notes created at top (max position + 1). POST /api/notes/reorder
  assigns positions from the given order (owner-scoped).
- notes store: position on Note, position-aware sort, optimistic reorder().
- NoteCard reorderable (native HTML5 draggable + dragstart/drop); BoardView moves
  the dragged note before the drop target and persists.

Note: drag on a CSS-columns masonry has imperfect during-drag visuals (columns
reflow); order persists correctly. Candidate for a polish pass / layout tweak.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FRgehjoz7Yv8LkUfADxACm

frontend/src/components/NoteCard.vue

@@ -5,8 +5,12 @@ import type { Note } from "../stores/notes";
 import Icon from "./Icon.vue";
 import NoteChecklist from "./NoteChecklist.vue";
 
-defineProps<{ note: Note }>();
-const emit = defineEmits<{ (e: "open", note: Note): void }>();
+defineProps<{ note: Note; reorderable?: boolean }>();
+const emit = defineEmits<{
+  (e: "open", note: Note): void;
+  (e: "dragstart", note: Note): void;
+  (e: "drop", note: Note): void;
+}>();
 const notes = useNotesStore();
 
 function cardClass(color: NoteColor): string {
@@ -18,6 +22,10 @@ function cardClass(color: NoteColor): string {
   <div
     class="group relative mb-4 break-inside-avoid rounded-xl border p-3 shadow-sm transition hover:shadow-md"
     :class="cardClass(note.color)"
+    :draggable="reorderable && !note.trashed"
+    @dragstart="emit('dragstart', note)"
+    @dragover.prevent
+    @drop="emit('drop', note)"
   >
     <img
       v-if="note.attachments.length"