feat: add sign parameter to upload_data for notary-signed uploads (#44)

[miles-on-nightshift]

Summary

Implements the sign parameter for the upload_data tool, resolving #44.

• gateway_client.py — adds optional sign: Optional[str] = None to upload_data(), passes it as a query param when provided
• server.py — adds sign to the MCP tool schema as an enum ["notary"] with descriptive help text; surfaces notary signer address in success response when present
• tests/test_gateway_client.py` — adds tests for both signed and unsigned upload paths

Behaviour

When sign="notary", the request becomes POST /api/v1/data/?sign=notary&stamp_id=..., triggering the gateway's notary service to cryptographically sign the data at upload time. The success response includes:

🎉 Data uploaded successfully to Swarm!

📄 Upload Details:
   Size: 42 bytes
   Content Type: application/json
   Stamp Used: `<stamp>`
   Signing: notary

🔗 Retrieval Information:
   Reference Hash: `<hash>`

🔐 Notary Signature:
   Signer: 0x...

When sign is omitted, behaviour is identical to the previous release.

Test plan

• pytest tests/test_gateway_client.py — 12 passed
• Manual: upload with sign="notary" against a dev gateway with notary enabled and confirm signature fields appear

Closes #44

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