Add migration 002: missing updated_at columns + auto-update triggers + indexes

Tables fixed:
- rooms              → added updated_at
- housekeeping_tasks → added updated_at
- channels           → added created_at + updated_at

Trigger function set_updated_at() attached to all 6 tables (hotels, users,
rooms, bookings, housekeeping_tasks, channels) — updated_at now auto-sets
on every UPDATE via BEFORE UPDATE trigger.

Indexes added for archiving / range queries:
- bookings(hotel_id, created_at), bookings(hotel_id, updated_at)
- housekeeping_tasks(hotel_id, created_at)
- rooms(hotel_id, updated_at)
- channels(hotel_id, updated_at)

Also patched 001_schema.sql so fresh installs start correct.
Migration runner is idempotent — skips already applied files.

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

backend/migrations/001_schema.sql

@@ -43,6 +43,7 @@ CREATE TABLE IF NOT EXISTS rooms (
   amenities       TEXT[]        NOT NULL DEFAULT '{}',
   notes           TEXT,
   created_at      TIMESTAMPTZ   NOT NULL DEFAULT NOW(),
+  updated_at      TIMESTAMPTZ   NOT NULL DEFAULT NOW(),
   UNIQUE(hotel_id, number)
 );
 
@@ -84,7 +85,8 @@ CREATE TABLE IF NOT EXISTS housekeeping_tasks (
   notes        TEXT,
   due_date     DATE,
   completed_at TIMESTAMPTZ,
-  created_at   TIMESTAMPTZ  NOT NULL DEFAULT NOW()
+  created_at   TIMESTAMPTZ  NOT NULL DEFAULT NOW(),
+  updated_at   TIMESTAMPTZ  NOT NULL DEFAULT NOW()
 );
 
 -- ── Channels ───────────────────────────────────────────────────────────────
@@ -97,6 +99,8 @@ CREATE TABLE IF NOT EXISTS channels (
   api_key_encrypted TEXT,
   hotel_external_id VARCHAR(255),
   last_synced_at    TIMESTAMPTZ,
+  created_at        TIMESTAMPTZ  NOT NULL DEFAULT NOW(),
+  updated_at        TIMESTAMPTZ  NOT NULL DEFAULT NOW(),
   UNIQUE(hotel_id, name)
 );