Cached, versioned extractor seam — prefactor for model-based extraction

[alexwolson]

This was generated by AI during triage.

Parent

#205

What to build

rebuild_agency_bids re-derives every agency_bid row from the held PDFs on each store pass — correct when derivation is regex and costs milliseconds, ruinous when it is a model call per document. This ticket adds a cache keyed on the document's sha256 plus a model+prompt version stamp, so a document that has not changed and whose extraction prompt has not changed is never re-extracted.

The existing regex derivation moves behind this seam unchanged. 800 tests green, zero behaviour change. A second extractor (the model path) becomes a drop-in behind it.

"Skip if rows exist" is the wrong guard — that is what makes stale rows persist. The version stamp is what makes re-extraction trigger on a prompt change, not on whether the table is empty.

Acceptance criteria

• A cache layer exists, keyed on (sha256, extractor_version), that short-circuits extraction when the key matches
• Existing regex-based derivation produces identical agency_bid rows through the new seam
• A changed extractor_version causes re-extraction on the next store pass
• 800 existing tests pass unchanged

Blocked by

None — can start immediately.