docs: expand README auth section for stdio and HTTP modes

[rahulcrl]

Summary

• Splits the Authentication section into stdio mode and HTTP mode subsections for clarity.
• Documents HTTP-specific env vars: CRDB_MCP_BEARER_TOKEN, CRDB_MCP_ALLOW_INSECURE_HTTP, and CRDB_MCP_ALLOW_NO_BEARER.
• Keeps the existing stdio guidance (cert-based recommended, password opt-in) intact.

Test plan

• Verify README renders correctly on GitHub.