Update Decision Settings

A sparse patch onto the stored policy. Absent keys stay unset.

The resolved capture policy, as the shared settings panel expects it.

Resolved rather than stored: the column holds only the keys somebody set, and the engine's resolver fills the rest from the preset on every read. So a repository never silently acquires a capture source because a default moved, and never silently keeps one either.

provider_available is what the repository's last index run actually had for model-backed capture, stamped by the indexer on repos.decision_capture -- the web process reads a different environment than the one the provider is built from, so it cannot answer this for itself. False when the last run had no provider, and also when the repository has not been indexed since the stamp existed; a warning says which. With it false, each source reports its deterministic stage running and its model stage skipped, with the reason.

etag is the concurrency token. The panel sends it back on write and a stale one is refused, so two people editing the same policy cannot silently overwrite each other.