Social login provider fallback workflow

[Wilfred007]

What

Social login provider fallback workflow.

Why

When a social auth provider is unavailable, there's no automated fallback workflow to an alternate login method.

Scope

In scope:

• Define and implement the workflow steps described against socialAuthController.ts
• Error handling and retry behavior for each step
• Observability (logging/tracing) for workflow execution
• Tests covering the happy path and at least one failure path

Out of scope:

• Redesigning the underlying service being orchestrated
• Building unrelated workflows not described in this issue
• New UI beyond what's needed to observe workflow status

Acceptance Criteria

• Workflow completes successfully on the happy-path fixture
• Workflow retries/handles at least one designed failure case
• Execution is observable via logs/traces
• Tests pass in CI

Technical Context

• Relevant files: socialAuthController.ts, frontend/src/components/SocialLoginButton.tsx
• New surface area: this is part of the Mastra AI integration (greenfield) — coordinate with whichever foundation/infra issues (agent runtime bootstrap, tool registry, auth scoping) have landed so far