perf(gradio): reduce UI initialization and refresh overhead

[cin-niko]

Description

This pull request introduces several improvements across the codebase, focusing mainly on concurrency configuration, UI responsiveness, event handling, and client-side logic for evidence search and PDF view. The most significant changes are the introduction of environment-based concurrency limits for file indexing, enhancements to evidence search initialization, and more robust event chaining and UI update logic.

Type of change

• New features (non-breaking change).
• Bug fix (non-breaking change).
• Breaking change (fix or feature that would cause existing functionality not to work as expected).

Checklist

• I have performed a self-review of my code.
• I have added thorough tests if it is a core feature.
• There is a reference to the original bug report and related work.
• I have commented on my code, particularly in hard-to-understand areas.
• The feature is well documented.