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/components/TaskCard.vue

@@ -19,7 +19,7 @@ const statusCycle: Record<TaskStatus, TaskStatus> = {
 };
 
 function cycleStatus() {
-  emit("status-toggle", props.task.id, statusCycle[props.task.status]);
+  emit("status-toggle", props.task.id, statusCycle[props.task.status!]);
 }
 
 function isOverdue(): boolean {
@@ -32,14 +32,14 @@ function isOverdue(): boolean {
   <router-link :to="`/tasks/${task.id}`" class="task-card">
     <div class="task-top">
       <StatusBadge
-        :status="task.status"
+        :status="task.status!"
         clickable
         @click.prevent.stop="cycleStatus"
       />
-      <PriorityBadge :priority="task.priority" />
+      <PriorityBadge :priority="task.priority!" />
       <h3 class="task-title">{{ task.title || "Untitled" }}</h3>
     </div>
-    <div v-if="task.description" class="task-preview prose" v-html="renderPreview(task.description)"></div>
+    <div v-if="task.body" class="task-preview prose" v-html="renderPreview(task.body)"></div>
     <div class="task-meta">
       <span v-if="task.due_date" :class="['due-date', { overdue: isOverdue() }]">
         Due: {{ task.due_date }}