refactor: remove EXTERNAL_BROWSERLESS_WS / browserless fallback

- Delete infrastructure/browser/ directory entirely (browser pool is gone)
- Remove browser pool initialization from bootstrap
- Remove fetch_via_browserless fallback from proxy_fetch
- perform_proxy_chain now only uses relay endpoints
- Clean up log messages, README, and compose env var
The proxy-bun service at https://git.imrnes.team/MythEclipse/proxy-bun
handles all proxy needs now.

src/infrastructure/mod.rs

@@ -1,4 +1,3 @@
-pub mod browser;
 pub mod cache;
 pub mod persistence;
 pub mod repository;