Backend: PUT /api/connections/{id} with UpdateConnectionRequest {Name, Config}.
Provider is immutable; a secret field left blank in the incoming config is
merged from the stored row so users can rename or rotate non-secret fields
without re-typing every credential. Invalidates the ConnectionInstanceRegistry
cache on update so the next GetConnection() re-opens with the fresh config.
409 on unique-name-per-provider collision, 404 on unknown id, 400 on non-
object config. 6 new xUnit tests cover the merge, overwrite, 404, 409, 400
paths plus the happy-path round-trip.
Frontend: ConnectionsPage gains an Edit pencil per row. The form is shared
between create and edit modes; provider select is disabled in edit; secret
fields render blank with an inline hint that leaving them blank keeps the
stored value. One new Vitest covers the edit flow.
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