Session 45: support email, audit fixes (template validation, content-type, admin auth, waitUntil)

- Added support@docfast.dev to footer, impressum, terms, landing page, openapi.json
- Fixed audit #6: Template render validates required fields (400 on missing)
- Fixed audit #7: Content-Type check on markdown/URL routes (415)
- Fixed audit #11: /v1/usage and /v1/concurrency now require ADMIN_API_KEY
- Fixed audit Critical #3: URL convert uses domcontentloaded instead of networkidle0

public/openapi.json

@@ -4,7 +4,7 @@
     "title": "DocFast API",
     "version": "1.0.0",
     "description": "Convert HTML, Markdown, and URLs to pixel-perfect PDFs. Built-in invoice & receipt templates.\n\n## Authentication\nAll conversion and template endpoints require an API key via `Authorization: Bearer <key>` or `X-API-Key: <key>` header.\n\n## Rate Limits\n- Free tier: 100 PDFs/month, 10 req/min\n- Pro tier: 10,000 PDFs/month, 30 req/min\n\n## Getting Started\n1. Sign up at [docfast.dev](https://docfast.dev) or via `POST /v1/signup/free`\n2. Verify your email with the 6-digit code\n3. Use your API key to convert documents",
-    "contact": { "name": "DocFast", "url": "https://docfast.dev" }
+    "contact": { "name": "DocFast", "url": "https://docfast.dev", "email": "support@docfast.dev" }
   },
   "servers": [{ "url": "https://docfast.dev", "description": "Production" }],
   "tags": [