feat: add ISR cache with stale-while-revalidate pattern

Implements in-memory caching for SSR pages using moka with:
- Configurable fresh/stale TTLs (60s/300s defaults)
- Background refresh for stale entries
- Cache invalidation on project/tag mutations
- Pre-cached icon collections on startup
- Skips cache for authenticated requests

.gitignore

@@ -1,4 +1,5 @@
 .env*
+/*.txt
 web/node_modules/
 target/
 .vscode/