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/types/note.ts

@@ -1,9 +1,16 @@
+export type TaskStatus = "todo" | "in_progress" | "done";
+export type TaskPriority = "none" | "low" | "medium" | "high";
+
 export interface Note {
   id: number;
   title: string;
   body: string;
   tags: string[];
   parent_id: number | null;
+  status: TaskStatus | null;
+  priority: TaskPriority | null;
+  due_date: string | null;
+  is_task: boolean;
   created_at: string;
   updated_at: string;
 }

frontend/src/types/task.ts

@@ -1,20 +1,6 @@
-export type TaskStatus = "todo" | "in_progress" | "done";
-export type TaskPriority = "none" | "low" | "medium" | "high";
-
-export interface Task {
-  id: number;
-  title: string;
-  description: string;
-  status: TaskStatus;
-  priority: TaskPriority;
-  due_date: string | null;
-  note_id: number | null;
-  tags: string[];
-  created_at: string;
-  updated_at: string;
-}
+export type { TaskStatus, TaskPriority, Note as Task } from "./note";
 
 export interface TaskListResponse {
-  tasks: Task[];
+  tasks: import("./note").Note[];
   total: number;
 }