Encoder parameter presets for monitor edit page

[connortechnology]

Add a per-encoder preset library to the monitor edit page so the user can pick recommended ffmpeg AVOptions for their selected encoder. Selecting a preset merges its params into the existing free-form EncoderParameters textarea (overwrite on conflict, keep user-only keys). A per-encoder valid-keys list drives advisory linting of unrecognised keys.

Spec: docs/superpowers/specs/2026-05-01-encoder-presets-design.md
Plan: docs/superpowers/plans/2026-05-01-encoder-presets.md

[IgorA100]

Spec: docs/superpowers/specs/2026-05-01-encoder-presets-design.md
Plan: docs/superpowers/plans/2026-05-01-encoder-presets.md

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