_do_rebuild_source was creating a fresh temp DB with only one source,
then atomically replacing analytics.duckdb — wiping views from every
other source. Now it delegates to _do_rebuild so all extract dirs are
re-attached in a single pass.
Adds test_rebuild_source_preserves_other_sources to guard the regression.
Extractors with remote tables now write a _remote_attach table into
extract.duckdb so the orchestrator can re-ATTACH external extensions
at query time. The mechanism is source-agnostic — any connector can use it.
- Keboola extractor writes _remote_attach + creates views on kbc.*
- Orchestrator reads _remote_attach, installs extension, reads token from env
- Graceful degradation: missing token → warning, local tables still work
