feat(hosting): add industry template infrastructure (Workstream 3B)

5 industry templates as JSON presets, each with theme + multi-page content:
- generic: clean minimal (homepage, about, contact)
- restaurant: warm tones, Playfair Display (homepage, about, menu, contact)
- construction: amber/earth tones, Montserrat (homepage, services, projects, contact)
- auto-parts: red/bold, parts-focused (homepage, catalog, contact)
- professional-services: navy/blue, Merriweather (homepage, services, team, contact)

Each template has:
- meta.json (name, description, tags, languages)
- theme.json (colors, fonts, layout, header style)
- pages/*.json (section-based homepage + content pages with i18n)
- {{placeholder}} variables for prospect data injection

TemplateService loads from templates_library/ directory with caching.
GET /admin/hosting/sites/templates endpoint to list available templates.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/modules/hosting/templates_library/professional-services/theme.json

@@ -0,0 +1,8 @@
+{
+  "theme_name": "modern",
+  "colors": {"primary": "#1e40af", "secondary": "#1e3a8a", "accent": "#3b82f6", "background": "#f8fafc", "text": "#0f172a", "border": "#cbd5e1"},
+  "font_family_heading": "Merriweather",
+  "font_family_body": "Source Sans Pro",
+  "layout_style": "grid",
+  "header_style": "fixed"
+}