Adds a nullable `aibridge_interception_error_type` enum and an
`error_message` column to `aibridge_interceptions`, so a failed
interception's terminal upstream error can be persisted.
Schema only: the write path and API exposure land in the stacked
backend PR.
*This PR was produced by opencode (agent) using the `anthropic/claude-opus-4-8` model, under human direction and review.*