docs: Update README — new backends, scenarios, API server, HuggingFace Spaces badge

[Neal006]

Background

The README is the project's front door — the first thing every contributor, researcher, and user sees. As v0.4 features land (GraphMemory, new scenarios, FastAPI server, HuggingFace Spaces), the README must stay in sync so visitors can discover, install, and use them immediately.

This is a documentation-only issue — no Python code needed. Perfect for a first contribution.

What to update

1. Badges section (top of README)

• Add a HuggingFace Spaces badge (placeholder link is fine until the Space is live):

  [![HuggingFace Spaces](https://img.shields.io/badge/%F0%9F%A4%97%20HuggingFace-Spaces-blue)](https://huggingface.co/spaces/Neal006/memorylens)

• Add a "Open Issues" badge:

  [![Open Issues](https://img.shields.io/github/issues/Neal006/memorylens)](https://github.com/Neal006/memorylens/issues)

2. Memory Backends table

Add a graph row:

Backend	Description	Token Cost	Best for
... existing rows ...
graph	Knowledge-graph with node deprecation on updates	Low	Structured fact tracking

3. Scenarios section (new or updated)

Add a table:

Scenario flag	Domain	Facts	Updates
--scenario default	Generic tech Q&A	8	2
--scenario edtech	Student-tutor	8	4
--scenario customer_support	Support ticket	8	2
--scenario medical	Patient consultation	8	2

4. REST API section (new)

## REST API

Start the API server:
```bash
pip install memorylens[api]
uvicorn api.server:app --reload

Endpoints:

• POST /benchmark — run a benchmark asynchronously
• GET /results/{run_id} — poll for results
• GET /docs — interactive Swagger UI

Example:

curl -X POST http://localhost:8000/benchmark \
  -H "Content-Type: application/json" \
  -d '{"backends": ["naive", "cascading"], "turns": 100}'
# → {"run_id": "abc12345", "status": "queued"}

5. CLI Reference section

Update the --backends example to include graph:

python main.py --backends naive rag graph cascading --turns 100

Add --scenario examples:

python main.py --scenario customer_support --turns 100
python main.py --scenario medical --seeds 5

6. Contributing section

Add a "Contributing" section that links to CONTRIBUTING.md and points to issues labeled good first issue.

Acceptance criteria

• GraphMemory listed in the backends table
• All 4 scenarios listed in a scenarios table
• API server section present with working code examples
• CLI reference updated with graph backend and new --scenario options
• HuggingFace Spaces badge present (even if placeholder)
• Contributing section links to CONTRIBUTING.md and to good first issue label
• All links in the README resolve correctly (use https:// not relative paths for external links)

Getting started

No coding required — this is a Markdown-only change.

# Verify CLI flags match what you document:
python main.py --help
python main.py --list-providers

Run the benchmark once to get real output and copy it into the README examples:

python main.py --backends naive cascading --turns 30