feat(mcp): operators can switch MCP (and OAuth) off

Admin switch in the system settings, MCP_ENABLED fixes it. Off: the
endpoint answers 404, OAuth is off, the API key settings hide the MCP tab.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/domain/models.go

@@ -418,6 +418,10 @@ type SystemSettings struct {
 	// update leaves it unchanged. RetentionNeverViaEnv: fixed by RETENTION_ALLOW_NEVER, not editable.
 	RetentionAllowNever  *bool `json:"retentionAllowNever,omitempty"`
 	RetentionNeverViaEnv bool  `json:"retentionNeverViaEnv,omitempty"`
+	// MCPEnabled: AI agents may connect over MCP (and claude.ai / ChatGPT over OAuth); nil in an update
+	// leaves it unchanged. MCPViaEnv: fixed by MCP_ENABLED, not editable.
+	MCPEnabled *bool `json:"mcpEnabled,omitempty"`
+	MCPViaEnv  bool  `json:"mcpConfiguredViaEnv,omitempty"`
 }
 
 // UnmarshalJSON accepts smtpPort as text or number: older clients sent a number, which made the whole