Replace startup compression with memory-serve

[meawoppl]

Summary

• Replace rust-embed + manual brotli/gzip startup compression with memory-serve
• Assets are now compressed at build time (brotli in release builds), not at server startup
• Eliminates the ~40s CPU spike on startup that was destabilizing shared hosts (Startup brotli compression pegs CPU for ~40s, can destabilize shared host #613)
• Removes rust-embed, brotli, flate2, mime_guess dependencies
• Deletes embedded_assets.rs (138 lines) — replaced by 6 lines of memory-serve config
• Built-in ETag/304, content negotiation, SPA fallback, cache-control headers

Closes #613

Test plan

• Backend starts instantly (no compression delay)
• Frontend loads at http://localhost:3000/
• SPA routing works (e.g. /dashboard returns index.html with 200)
• Assets have correct Content-Encoding headers (brotli/gzip in release)
• Cache-Control: no-cache on index.html, immutable on hashed assets