feat: add favicon.ico and gate model write APIs by env flag

- Generate and serve real /favicon.ico from static assets
- Update HTML to use /favicon.ico
- Add ENABLE_MODEL_RW_API setting (default: false)
- Disable POST/DELETE model endpoints by default
- Hide write endpoints from OpenAPI when disabled
- Return 404 for write endpoints when disabled
- Update env.example with ENABLE_MODEL_RW_API documentation
- Update README and PRD with R/W API policy and remote compose notes
- Add tests to verify write endpoints are disabled by default

app/web/templates/index.html

@@ -4,7 +4,7 @@
     <meta charset="UTF-8">
     <meta name="viewport" content="width=device-width, initial-scale=1.0">
     <title>LLM Monitor - Dashboard Ollama</title>
-    <link rel="icon" type="image/svg+xml" href="data:image/svg+xml,<svg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 100 100'><text y='.9em' font-size='90'>🦙</text></svg>">
+    <link rel="icon" href="/favicon.ico" sizes="any">
     <!-- Tailwind CSS (compiled for production) -->
     <link rel="stylesheet" href="/static/css/output.css">
     <!-- Fallback CDN for development (if output.css not available) -->