v0.11.12 new migration to fix orphaned users

backend/src/models/migrations/006_scrub_deleted_users.sql

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+-- Migration 006: Scrub pre-existing deleted users
+--
+-- Prior to v0.11.11, deleting a user only set status='deleted' — the original
+-- email, name, avatar, and messages were left untouched. This meant:
+--   • The email address was permanently blocked from re-use
+--   • Message content was still stored and attributable
+--   • Direct messages were left in an inconsistent half-alive state
+--
+-- v0.11.11 introduced proper anonymisation in the delete route, but that only
+-- applies to users deleted from that point forward. This migration back-fills
+-- the same treatment for any users already sitting in status='deleted'.
+--
+-- Data mutation note: the MIGRATIONS.md convention discourages data changes in
+-- migrations. This is a deliberate exception — the whole point of this migration
+-- is to correct orphaned rows that cannot be fixed any other way. The UPDATE
+-- statements are all guarded by WHERE status='deleted' so they are safe to
+-- replay against schemas that are already clean.
+
+-- ── 1. Anonymise deleted user records ────────────────────────────────────────
+-- Scrub email to deleted_{id}@deleted to free the address for re-use.
+-- Only touch rows where the email hasn't already been scrubbed (idempotent).
+UPDATE users
+SET
+  email        = 'deleted_' || id || '@deleted',
+  name         = 'Deleted User',
+  display_name = NULL,
+  avatar       = NULL,
+  about_me     = NULL,
+  password     = '',
+  updated_at   = NOW()
+WHERE status = 'deleted'
+  AND email NOT LIKE 'deleted\_%@deleted' ESCAPE '\';
+
+-- ── 2. Anonymise their messages ───────────────────────────────────────────────
+-- Mark all non-deleted messages from deleted users as deleted so they render
+-- as "This message was deleted" rather than remaining attributable.
+UPDATE messages
+SET
+  is_deleted = TRUE,
+  content    = NULL,
+  image_url  = NULL
+WHERE is_deleted = FALSE
+  AND user_id IN (SELECT id FROM users WHERE status = 'deleted');
+
+-- ── 3. Freeze their direct messages ──────────────────────────────────────────
+-- Any 1:1 DM involving a deleted user becomes read-only. The surviving member
+-- keeps their history but can no longer send into a dead conversation.
+UPDATE groups
+SET
+  is_readonly = TRUE,
+  updated_at  = NOW()
+WHERE is_direct = TRUE
+  AND is_readonly = FALSE
+  AND (
+    direct_peer1_id IN (SELECT id FROM users WHERE status = 'deleted')
+    OR
+    direct_peer2_id IN (SELECT id FROM users WHERE status = 'deleted')
+  );