feat: add credential substitution with global credentials support

[tito]

Summary

End-to-end credential substitution: real API keys never reach sandboxed processes. The proxy transparently replaces opaque placeholders with real values before forwarding HTTP requests upstream.

Global credentials

Global credentials are stored in the dashboard and injected on demand via --inject:

greywall --inject ANTHROPIC_API_KEY --inject OPENAI_API_KEY -- opencode

The session creation API (POST /api/sessions) accepts a global_credentials field (list of labels). The proxy resolves each label, merges placeholder-to-value mappings into the session, and returns the placeholders so greywall can set them as environment variables.

Example request:

{
  "session_id": "gw-abc123",
  "container_name": "opencode",
  "global_credentials": ["ANTHROPIC_API_KEY"],
  "ttl_seconds": 900
}

Example response:

{
  "session_id": "gw-abc123",
  "expires_at": "2026-03-25T23:00:00Z",
  "credential_count": 1,
  "global_credentials": {
    "ANTHROPIC_API_KEY": "greyproxy:credential:v1:global:a1b2c3..."
  }
}

Substitution tracking

• Substitution counts flushed to DB every 60s, broadcast via WebSocket (session.substitution event)
• Activity and traffic tables show shield icon for substituted requests, credential labels as badges in expanded details
• Session cards show creation time and active duration

Changes

• API: POST /api/sessions accepts global_credentials, resolves labels, returns placeholders
• Credential store: publishes session.substitution events after flushing counts
• Activity: ActivityItem includes SubstitutedCredentials; shield icon and label badges in both activity and traffic tables
• Settings UI: global credentials section explains --inject workflow and substitution behavior
• Tests: CRUD, API handler, and end-to-end substitution tests for global credentials
• Docs: docs/credential-substitution.md covering the full API, substitution behavior, and dashboard UI

Test plan

• go test ./internal/greyproxy/... passes
• go test ./internal/greyproxy/api/... passes (session creation tests)
• greywall --inject LABEL -- env shows placeholder in output
• HTTP request with placeholder substituted (shield icon in activity view)
• Settings > Credentials > Active Sessions shows substitution count updating

Dependencies

Companion greywall PR: GreyhavenHQ/greywall#63