[EPAC-2304]: Lock bills SQLite artifact schema with a producer-to-consumer seam test

[riddim-developer-bot]

Scope

The bills SQLite artifact is an implicit contract between two separate Go binaries: the bills-indexer writer (producer, backend/bills-indexer/internal/adapter/sqlite/writer.go) and the bills serving repository (consumer, backend/bills/internal/adapter/sqlite/repository.go). Nothing fast verified the assembled contract — only the staging smoke did, which cannot catch drift before deploy. The serving repo also masked drift with dynamic column-name fallbacks (columnExpr/firstColumn/…), so as shipped the served BillVersion's title/chamber/published_on were always empty (the indexer never writes those columns) and the version label silently fell back to stage.

This PR locks that contract:

• Seam test (backend/bills/artifact_seam_test.go, TestBillsArtifactSeam). It drives the real producer binary in a new offline fixture mode to write a real bills.db, then opens that file with the real serving repository and asserts ListBills, GetBillDepth, and GetBillVersionDiff return the promised columns populated — no NULL fallbacks. The two adapters are internal to separate Go modules, so (per the issue's guidance) neither imports the other's internals; the only thing shared across the seam is the on-disk SQLite file, exactly as in production.
• Single-sourced bill_versions contract. The served BillVersion is now exactly {id, label, stage, source_url}, where label and stage both carry the LEGISinfo publication-type name (the indexer has no separate label column) and source_url is the indexer's html_url. The always-empty title, chamber, and published_on were dropped from the domain model and backend/openapi/openapi.json (schema + examples). The decision is recorded in docs/architecture/bills-artifact-contract-epac2304.md.
• Dead fallbacks removed. The version and amendment reads now use fixed SQL against the producer's real columns; the columnExpr/firstColumn/tableColumns/orderExpr helpers are gone.
• Offline producer mode (BILLS_FIXTURE_BATCH) added to the bills-indexer so the seam test can drive the real writer without LEGISinfo or AWS. The deployed pipeline never sets it.
• CI gate. The bills module is now run in pr-build's backend-tests job, so schema drift fails the required gate instead of in staging smoke or production.

No runtime response change / no Release-Note: the dropped version fields were already omitted from responses (always empty, omitempty), and iOS decodes every version field as optional. The served JSON is byte-identical; this is an API-spec + test/refactor change with no user-visible effect.

Out of scope (documented follow-up): the served BillAmendment over-promises fields the indexer never writes; the consumer now reads only the producer-backed id/source_url (identical to today's production output) with fixed SQL, and the remaining contract trim/enrich is captured in the "Known limitation" section of the architecture note.

Bugfix SPEC

• Spec: N/A — architecture/test hardening, not a bug fix.
• Trace ID: N/A

Testing notes

• Automated tests run: go test ./backend/bills/... ./backend/bills-indexer/... (pass), go test -tags=integration ./... for bills (pass), go test ./... for the openapi module (pass), gofmt/go vet clean, actionlint on the workflow (pass).
• Manual verification: N/A — backend-only change, no UI surface. The seam test is the verification; it builds a real artifact and reads it back through the serving repository.

Screenshots

N/A — no UI change.

Related issue

• Closes: https://linear.app/riddimsoftware/issue/EPAC-2304/lock-bills-sqlite-artifact-schema-with-a-producer-to-consumer-seam