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

src/thoughtsync/models/note.py

@@ -3,7 +3,7 @@ from __future__ import annotations
 import uuid
 from datetime import datetime
 
-from sqlalchemy import Boolean, DateTime, ForeignKey, Index, Text, func
+from sqlalchemy import Boolean, DateTime, ForeignKey, Index, Integer, Text, func
 from sqlalchemy.dialects.postgresql import UUID
 from sqlalchemy.orm import Mapped, mapped_column
 
@@ -42,6 +42,8 @@ class Note(Base):
     color: Mapped[str] = mapped_column(Text(), nullable=False, server_default="default")
     # 'text' (freeform body) or 'list' (a checklist of note_items).
     kind: Mapped[str] = mapped_column(Text(), nullable=False, server_default="text")
+    # Manual drag order (higher = earlier); 0 until the user reorders.
+    position: Mapped[int] = mapped_column(Integer(), nullable=False, server_default="0")
     pinned: Mapped[bool] = mapped_column(Boolean(), nullable=False, server_default=func.false())
     archived: Mapped[bool] = mapped_column(Boolean(), nullable=False, server_default=func.false())
     # Soft delete: non-null => in Trash. Restore sets it back to null.
@@ -58,6 +60,7 @@ class Note(Base):
             "body": self.body,
             "color": self.color,
             "kind": self.kind,
+            "position": self.position,
             "pinned": self.pinned,
             "archived": self.archived,
             "trashed": self.deleted_at is not None,