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/notes.ts

@@ -142,17 +142,27 @@ export const useNotesStore = defineStore("notes", () => {
     return await apiPost<Note>("/api/notes/resolve-title", { title });
   }
 
-  async function convertToTask(noteId: number) {
-    const task = await apiPost<Record<string, unknown>>(
+  async function convertToTask(noteId: number): Promise<Note> {
+    const note = await apiPost<Note>(
       `/api/notes/${noteId}/convert-to-task`,
       {}
     );
-    // Remove the note from local state since it was converted
-    notes.value = notes.value.filter((n) => n.id !== noteId);
+    // Update local state — the note is now a task
     if (currentNote.value?.id === noteId) {
-      currentNote.value = null;
+      currentNote.value = note;
     }
-    return task;
+    return note;
+  }
+
+  async function convertToNote(noteId: number): Promise<Note> {
+    const note = await apiPost<Note>(
+      `/api/notes/${noteId}/convert-to-note`,
+      {}
+    );
+    if (currentNote.value?.id === noteId) {
+      currentNote.value = note;
+    }
+    return note;
   }
 
   async function fetchBacklinks(
@@ -196,6 +206,7 @@ export const useNotesStore = defineStore("notes", () => {
     refresh,
     resolveTitle,
     convertToTask,
+    convertToNote,
     fetchBacklinks,
     fetchAllTags,
   };