fix: make SSE buffer size configurable to prevent overflow during multi-round searches (#664) (#665)

* fix: make SSE buffer size configurable to prevent overflow during multi-round searches (Issue #664)

- Add NEXT_PUBLIC_MAX_STREAM_BUFFER_SIZE environment variable for frontend SSE stream buffer
- Default to 1MB for backward compatibility, users can increase to 5-10MB for large searches
- Enhance error message with actual buffer sizes and guidance on configuration
- Add validation schema in env.js with positive integer requirement
- Document configuration in .env.example with clear examples and use cases

* Apply suggestions from code review

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

web/src/env.js

@@ -23,6 +23,7 @@ export const env = createEnv({
   client: {
     NEXT_PUBLIC_API_URL: z.string().optional(),
     NEXT_PUBLIC_STATIC_WEBSITE_ONLY: z.boolean().optional(),
+    NEXT_PUBLIC_MAX_STREAM_BUFFER_SIZE: z.coerce.number().int().positive().optional(),
   },
 
   /**
@@ -34,6 +35,7 @@ export const env = createEnv({
     NEXT_PUBLIC_API_URL: process.env.NEXT_PUBLIC_API_URL,
     NEXT_PUBLIC_STATIC_WEBSITE_ONLY:
       process.env.NEXT_PUBLIC_STATIC_WEBSITE_ONLY === "true",
+    NEXT_PUBLIC_MAX_STREAM_BUFFER_SIZE: process.env.NEXT_PUBLIC_MAX_STREAM_BUFFER_SIZE,
     AMPLITUDE_API_KEY: process.env.AMPLITUDE_API_KEY,
     GITHUB_OAUTH_TOKEN: process.env.GITHUB_OAUTH_TOKEN,
   },