feat: seed default MCP servers (Findings, SBOM, DAST) on dashboard startup

- Add MCP_ENDPOINT_URL env var to configure MCP server base URL
- Seed three default MCP server configs on dashboard startup if not present
- Each server has its own tool subset: findings (3 tools), SBOM (2 tools), DAST (2 tools)
- Uses upsert-by-name to avoid duplicates on restart

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

compliance-core/src/config.rs

@@ -35,4 +35,5 @@ pub struct DashboardConfig {
     pub mongodb_database: String,
     pub agent_api_url: String,
     pub dashboard_port: u16,
+    pub mcp_endpoint_url: Option<String>,
 }