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

alembic/versions/0008_note_position.py

@@ -0,0 +1,21 @@
+"""notes.position for manual drag-reorder
+
+Revision ID: 0008
+Revises: 0007
+Create Date: 2026-07-20
+"""
+from alembic import op
+import sqlalchemy as sa
+
+revision = "0008"
+down_revision = "0007"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.add_column("notes", sa.Column("position", sa.Integer(), nullable=False, server_default="0"))
+
+
+def downgrade() -> None:
+    op.drop_column("notes", "position")