Add API key authentication for local and hosted memory endpoints

[aiprofithub]

Add a minimal API key auth layer for MemoryMesh.

Goal:

• make the API feel closer to a real hosted product
• support future Cloud Preview usage limits
• keep local development simple

Suggested scope:

• accept Authorization: Bearer
• allow auth to be disabled in local dev
• return clear 401 errors
• add a short README example

Out of scope for v0:

• full user management
• billing
• team roles
• enterprise SSO