feat: add resilient transcription fallback chains

[GodsQuantum]

Adds ordered provider+model fallback chains, live model discovery, optional per-route fallback timeouts, durable attempt history, backward compatibility, README documentation, and deterministic E2E coverage.

Highlights:

• freely mix providers and models, including multiple models from the same provider
• no processing timeout by default for long audio
• cancellation is terminal; provider/transport failures can fall through
• Quick Transcribe and Workflows share the same chain editor
• Jobs show route attempt history and the provider/model that actually succeeded
• same-provider/different-model fallback is covered by E2E tests

Local full gate: 47 Rust tests + 15 frontend tests, Svelte check, build, npm audit, E2E, public privacy check.