Add stale-scrape health alerting (email + in-app banner)

Silent scraper breakage (200-with-empty-body) went unnoticed for 16 days
because nothing flagged it. Now, after each daily direct scrape, a health
check compares every enabled competitor's freshest captured rate against a
staleness threshold (default 5 days, config: scrape_health_stale_days).

State lives in a new direct_scrape_health table so we alert on transitions
only — one email when a competitor goes stale, one when it recovers, never a
daily repeat. Email uses the stack's shared SMTP integration via
central_settings (recipient: scrape_health_alert_email, else SMTP reply_to/
from) — no new mail secrets in this app. The current stale set also drives an
in-app warning banner on the Direct Rates page (GET /direct/health), with an
on-demand re-check endpoint (POST /direct/health/check).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/schema.sql

@@ -331,6 +331,19 @@ CREATE INDEX IF NOT EXISTS idx_direct_rates_scraped    ON direct_rates(scraped_a
 ALTER TABLE booking_com_hotels ADD COLUMN IF NOT EXISTS direct_hotel_id INTEGER
     REFERENCES direct_competitor_hotels(id) ON DELETE SET NULL;
 
+-- ============================================
+-- DIRECT SCRAPE HEALTH (per-hotel freshness state, drives stale-data alerting)
+-- ============================================
+
+CREATE TABLE IF NOT EXISTS direct_scrape_health (
+    hotel_id     INTEGER PRIMARY KEY REFERENCES direct_competitor_hotels(id) ON DELETE CASCADE,
+    state        TEXT NOT NULL DEFAULT 'ok',   -- 'ok' | 'stale'
+    fresh_as_of  TIMESTAMPTZ,                  -- freshest direct_rates.scraped_at seen for this hotel
+    stale_since  TIMESTAMPTZ,                  -- when it first went stale (cleared on recovery)
+    notified_at  TIMESTAMPTZ,                  -- last time a transition email was sent
+    updated_at   TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
 -- Pin the Booking.com destination: free-text ss= searches non-deterministically
 -- resolve to the wrong place (Stow on the Wold once matched St. Wolfgang, AT).
 ALTER TABLE booking_scrape_config ADD COLUMN IF NOT EXISTS dest_id VARCHAR(32);