Add DELETE /api/auth/account endpoint for full account deletion

Implements task 6.3: deletes all user data in correct FK order
(span_annotations, annotations, candles, charts, span_label_types,
annotation_types) then deletes the user record. Returns 401 if not
authenticated, 200 on success.

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

openspec/changes/user-accounts/tasks.md

@@ -33,7 +33,7 @@
 
 - [x] 6.1 `[haiku]` Create `PUT /api/auth/profile` endpoint: update user display name
 - [x] 6.2 `[sonnet]` Create `PUT /api/auth/password` endpoint: verify current password, hash new password, update; reject for OAuth users
-- [ ] 6.3 `[sonnet]` Create `DELETE /api/auth/account` endpoint: delete all user data (cascade) and user record
+- [x] 6.3 `[sonnet]` Create `DELETE /api/auth/account` endpoint: delete all user data (cascade) and user record
 
 ## 7. Update Existing API Routes
 

src/app/api/auth/account/route.ts

@@ -0,0 +1,54 @@
+import { NextResponse } from 'next/server';
+import { eq } from 'drizzle-orm';
+import { db } from '@/lib/db';
+import {
+  users,
+  spanAnnotations,
+  annotations,
+  candles,
+  charts,
+  spanLabelTypes,
+  annotationTypes,
+} from '@/lib/db/schema';
+import { getAuthUser } from '@/lib/auth';
+
+export async function DELETE() {
+  // Get authenticated user
+  const user = await getAuthUser();
+  if (!user) {
+    return NextResponse.json({ error: 'Unauthorized' }, { status: 401 });
+  }
+
+  const userId = user.id;
+
+  // Delete in FK-safe order:
+  // 1. span_annotations (references charts)
+  await db.delete(spanAnnotations).where(eq(spanAnnotations.user_id, userId));
+
+  // 2. annotations (references charts)
+  await db.delete(annotations).where(eq(annotations.user_id, userId));
+
+  // 3. candles (references charts — no user_id, must go via chart_id)
+  const userCharts = await db
+    .select({ id: charts.id })
+    .from(charts)
+    .where(eq(charts.user_id, userId));
+
+  for (const chart of userCharts) {
+    await db.delete(candles).where(eq(candles.chart_id, chart.id));
+  }
+
+  // 4. charts
+  await db.delete(charts).where(eq(charts.user_id, userId));
+
+  // 5. span_label_types (references users)
+  await db.delete(spanLabelTypes).where(eq(spanLabelTypes.user_id, userId));
+
+  // 6. annotation_types (references users)
+  await db.delete(annotationTypes).where(eq(annotationTypes.user_id, userId));
+
+  // 7. user record
+  await db.delete(users).where(eq(users.id, userId));
+
+  return NextResponse.json({ message: 'Account deleted successfully' }, { status: 200 });
+}