Merge tasks into notes: a task is just a note with task attributes

A task is now a note with status/priority/due_date columns set (status IS NOT NULL).
This eliminates the separate tasks table, companion note system, cascade deletes,
bidirectional title sync, and _skip_cascade flags.

Migration (0004):
- Add status, priority, due_date columns to notes table
- Migrate task data from companion notes and orphan tasks
- Drop tasks table and old enum types

Backend:
- models/note.py: Add TaskStatus/TaskPriority enums, task columns, is_task property
- models/task.py: Deleted (merged into note.py)
- models/__init__.py: Re-export enums from note.py, remove Task import
- services/notes.py: Remove companion/cascade logic, add is_task filter,
  convert_note_to_task, convert_task_to_note, simplified backlinks
- services/tasks.py: Rewritten as thin wrappers around notes service
- routes/notes.py: Add is_task filter (default false), task fields in CRUD,
  convert-to-note endpoint
- routes/tasks.py: description→body (with fallback), remove note_id filter

Frontend:
- types/note.ts: Add TaskStatus, TaskPriority, task fields to Note interface
- types/task.ts: Task is now a re-export alias for Note
- stores/notes.ts: Simplify convertToTask, add convertToNote
- stores/tasks.ts: description→body in createTask/updateTask
- TaskEditorView: description→body, remove companion note UI
- TaskViewerView: description→body, remove companion note link, add Convert to Note
- NoteViewerView: Remove companion task UI, simplify convert-to-task
- TaskCard: description→body, non-null assertions for status/priority

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/stores/tasks.ts

@@ -57,7 +57,7 @@ export const useTasksStore = defineStore("tasks", () => {
 
   async function createTask(data: {
     title: string;
-    description: string;
+    body: string;
     status?: TaskStatus;
     priority?: TaskPriority;
     due_date?: string | null;
@@ -68,7 +68,7 @@ export const useTasksStore = defineStore("tasks", () => {
   async function updateTask(
     id: number,
     data: Partial<
-      Pick<Task, "title" | "description" | "status" | "priority" | "due_date">
+      Pick<Task, "title" | "body" | "status" | "priority" | "due_date">
     >
   ): Promise<Task> {
     const task = await apiPut<Task>(`/api/tasks/${id}`, data);