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>

Dockerfile.railway

@@ -74,7 +74,8 @@ ENV AUTH_TOKEN="REPLACE_THIS_AUTH_TOKEN_32_CHARS_MIN_abcdefgh"
 ENV NODE_ENV=production
 ENV IS_DOCKER=true
 ENV MCP_MODE=http
-ENV USE_FIXED_HTTP=true
+# NOTE: USE_FIXED_HTTP is deprecated and removed. SingleSessionHTTPServer is now the default.
+# See: https://github.com/czlonkowski/n8n-mcp/issues/524
 ENV LOG_LEVEL=info
 ENV TRUST_PROXY=1
 ENV HOST=0.0.0.0