feat: Cascade reuse optimization for single-user setups & HTTPS proxy

[Fermiz]

Summary

This PR improves cascade conversation reuse for personal/single-user setups (e.g., Claude Code, Claude Desktop, Claude for Office plugins) and adds an HTTPS reverse proxy for clients that require TLS.

Problem

When using WindsurfAPI with Claude Code in a single-user setup:

1. Cascade creates multiple conversations instead of reusing the same one, because the conversation fingerprint drifts between turns (system prompt changes with timestamps, cwd, git status, etc.)
2. Office plugins (e.g., Claude for PowerPoint/Word) require HTTPS but WindsurfAPI only serves HTTP
3. Pool size is hardcoded at 500 entries — wasteful for single-user setups

Changes

Caller-based cascade reuse fallback

• New env var CASCADE_REUSE_BY_CALLER=1: adds a secondary index (callerKey + model) -> cascade in the conversation pool
• When the primary fingerprint lookup misses (due to system prompt drift), falls back to the latest cascade for the same caller+model
• Result: single-user Claude Code sessions always reuse the same cascade ID, avoiding redundant history replays

Configurable pool size

• New env var CASCADE_POOL_MAX=N (default 500): set to 1-5 for single-user setups to minimize memory usage

HTTPS reverse proxy

• https-proxy.js: lightweight HTTP/2 + HTTP/1.1 reverse proxy with full CORS support
• Handles Office plugin requirements (HTTPS, HTTP/2, preflight headers)
• Uses mkcert for locally-trusted certificates

Documentation

• local-setup.md: step-by-step guide for HTTPS setup with mkcert
• Updated .env.example, README.md, README.en.md with new env vars

Recommended .env for single-user personal use

CASCADE_REUSE_BY_CALLER=1
CASCADE_POOL_MAX=1

Files changed

• src/conversation-pool.js — configurable pool max, caller-based fallback index
• src/handlers/chat.js — pass modelKey to pool checkout/checkin
• https-proxy.js — new HTTPS reverse proxy
• local-setup.md — setup guide
• .env.example, .gitignore, README.md, README.en.md — docs

Testing

• All 34 existing tests pass
• Verified HTTPS proxy with Claude Messages API
• Verified with Claude for PowerPoint plugin (HTTP/2 + CORS)