`_init_tables` left `chunks.id`, `chunks.document_id` and `documents.id`
unindexed, so those lookups scanned the column. On object storage that is
network I/O per query, on paths that run per document: citation lookup,
delete-by-document, the re-ingest merge, and every dedup probe.
Declare the index set per table in `index_specs()` and apply it through
`ensure_indexes()`, which skips a column only when it is already indexed with
the declared type. Both halves of that are load-bearing. Skipping is required
because `create_index(replace=True)` rebuilds an identical index, writing a new
index and a new table version and orphaning the old files until the next vacuum.
Comparing the type is required because column coverage alone would let a
wrong-typed index stand, and a BTree on `label` silently loses the
low-cardinality equality lookup the Bitmap is there for.
Columns not declared for a table are left alone, so an externally created index
such as a vector index on `chunks` survives.
`_init_tables`, `recreate_embeddings_table`, `ChunkRepository.delete_all` and
`DocumentRepository.delete_all` now all route through it instead of repeating
their own subsets.
Also recreate `document_items` from `get_document_items_arrow_schema()` in
`DocumentRepository.delete_all`, which was using the LanceModel and so returned
`picture_data` as 32-bit `binary`.
Existing databases are unchanged; the migration follows separately.
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