feat: port signing fix, tool emulation, retry, circuit breaker, UI + docs

[mingles-agent]

Closes #1 in gonka-proxy-service.

What's changed

1. Signing fix — app/gonka_client.py

• Added encode_with_low_s() as a standalone function for clean low-S ECDSA normalization
• Added stream_read_timeout parameter (default 300s) to avoid stream cutoffs on long generations
• Updated _sign_payload to use Phase 3 signing: SHA-256 hash of payload → input for signature (matches private repo)
• No multi-node/DB/cluster code ported

2. Tool emulation — app/tool_emulation.py

• Full port of tool emulation module (no DB dependencies)
• Supports JSON format ({"tool_calls": [...]}) and XML format (<tool_call>...</tool_call>)
• Handles both streaming and non-streaming response paths
• Also ports test_tool_emulation.py with adapted imports

3. Retry — app/retry.py

• Full port of retry utilities with exponential backoff + jitter
• retry_with_backoff, retry_with_backoff_args, RetryConfig, predefined configs

4. Circuit breaker — app/circuit_breaker.py

• Full port of circuit breaker implementation
• CLOSED → OPEN after 5 failures, 60s recovery timeout, HALF_OPEN after timeout

5. UI — app/static/index.html

• Updated UI from private repo
• All mingles.ai references replaced with localhost:8000

6. Wired into main.py

• Tool emulation applied transparently: if tools in request → emulate, parse response back to OpenAI format
• Circuit breaker + retry wrap non-streaming GonkaClient calls
• GONKA_STREAM_READ_TIMEOUT env var exposed in Settings
• /health endpoint now includes circuit breaker state

7. README.md

• Complete rewrite: self-hosted setup, all env vars with descriptions, Docker run example
• Removed 👉 https://gonka-gateway.mingles.ai/ line
• No mingles.ai references anywhere in the repo

Acceptance criteria

• ✅ Starts with minimal .env (no MongoDB): GONKA_PRIVATE_KEY, GONKA_ADDRESS, GONKA_ENDPOINT, GONKA_PROVIDER_ADDRESS, API_KEY
• ✅ Tool emulation wired in: falls back to prompt-based emulation if model doesn't support tools natively
• ✅ Circuit breaker and retry wrap GonkaClient calls
• ✅ README clean and sufficient for self-hosted setup
• ✅ No mingles.ai URLs in public repo