docs: add missing SSE provider event and full status/error field shapes

[detail-app]

The SSE event types list was missing the 'provider' event type that the
backend emits during fallback scenarios, and the status/error event
shapes only documented message/details, omitting the provider, code,
retryable, stage, attempt, and maxAttempts fields defined in the
canonical Zod schemas and validated by the test suite.

A developer implementing an SSE consumer based solely on the docs would
not know to handle 'provider' events, and would miss that status events
carry provider attribution for fallback notifications.

Introduced by commit d8f8183 (@WilliamAGH, #77)

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