feat: production hardening + smart subpage scanning with layout dedup

Security:
- Add CRON_SECRET auth to /api/cron/* endpoints
- Add admin role verification to /api/admin/* routes
- Add org membership check to /api/billing/usage
- Add security headers (HSTS, X-Frame-Options, CSP, etc.)
- Add env variable validation at startup
- Add rate limiting to backend API (30 req/min per IP)

Infrastructure:
- Multi-stage Dockerfiles with non-root user + healthchecks
- Updated cron workflow to pass CRON_SECRET header
- Updated .env.example with all optional vars

Smart subpage scanning:
- Crawler now computes template_hash (DOM structure without content)
- Scanner scans ALL unique-layout pages, not just main page
- Pages with same layout (e.g. product pages) scanned only once
- Deduplication by template_hash, fallback to content_hash
- Main page always scanned with high priority
- Re-checks subscription limits before each page scan

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

website-monitoring-frontend/database-schema.sql

@@ -203,4 +203,8 @@ CREATE TABLE IF NOT EXISTS alert_configurations (
   created_at timestamp with time zone DEFAULT now(),
   updated_at timestamp with time zone DEFAULT now()
 );
+
+-- Add template_hash to pages table for layout deduplication
+ALTER TABLE pages ADD COLUMN IF NOT EXISTS template_hash VARCHAR;
+CREATE INDEX IF NOT EXISTS idx_pages_template_hash ON pages(template_hash) WHERE template_hash IS NOT NULL;
   );