fix: deprecate USE_FIXED_HTTP for SSE streaming support (Issue #524) (#525)

* fix: deprecate USE_FIXED_HTTP for SSE streaming support (Issue #524)

The fixed HTTP implementation does not support SSE streaming required
by clients like OpenAI Codex. This commit deprecates USE_FIXED_HTTP
and makes SingleSessionHTTPServer the default.

Changes:
- Add deprecation warnings in src/mcp/index.ts and src/http-server.ts
- Remove USE_FIXED_HTTP from docker-compose.yml and Dockerfile.railway
- Update .env.example with deprecation notice
- Rename npm script to start:http:fixed:deprecated
- Update all documentation to remove USE_FIXED_HTTP references
- Mark test case as deprecated

Users should unset USE_FIXED_HTTP to use the modern SingleSessionHTTPServer
which supports both JSON-RPC and SSE streaming.

Closes #524

Concieved by Romuald Członkowski - www.aiadvisors.pl/en

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* chore: bump version to 2.31.8 and add CHANGELOG entry

- Fix comment inaccuracy: "deprecated" not "deprecated and removed"
- Bump version from 2.31.7 to 2.31.8
- Add CHANGELOG entry documenting USE_FIXED_HTTP deprecation
- Update all deprecation messages to reference v2.31.8

Concieved by Romuald Członkowski - www.aiadvisors.pl/en

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

---------

Co-authored-by: Romuald Członkowski <romualdczlonkowski@MacBook-Pro-Romuald.local>
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

docker-compose.yml

@@ -12,7 +12,8 @@ services:
     environment:
       # Mode configuration
       MCP_MODE: ${MCP_MODE:-http}
-      USE_FIXED_HTTP: ${USE_FIXED_HTTP:-true}  # Use fixed implementation for stability
+      # NOTE: USE_FIXED_HTTP is deprecated. SingleSessionHTTPServer is now the default.
+      # See: https://github.com/czlonkowski/n8n-mcp/issues/524
       AUTH_TOKEN: ${AUTH_TOKEN:?AUTH_TOKEN is required for HTTP mode}
       
       # Application settings