feat(pseo): create pSEO Engine admin templates + sidebar nav
- base_admin.html: add pSEO section with "pSEO Engine" link
- pseo_dashboard.html: template stats, freshness badges, HTMX gaps panels,
recent jobs table, health issues HTMX-loaded section
- pseo_health.html: HTMX partial — hreflang orphans, missing build files,
broken scenario refs (collapsible details with drill-down tables)
- pseo_gaps.html: HTMX partial — missing rows per template with generate button
- pseo_jobs.html: full jobs list with live progress bars (HTMX polling)
- pseo_job_status.html: HTMX partial — polls every 2s while job is pending
- app.py: add `fromjson` Jinja2 filter for displaying task payloads
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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