feat(parties): phase 0 prerequisites, pg_trgm and golden-set draw (#2157)

* feat(parties): phase 0 prerequisites, pg_trgm and golden-set draw

Phase 0 of the Kontakter plan: make the counterparty resolver measurable
before building it.

- Migration 20260902120000 enables pg_trgm (trigram blocking of
  counterparty keys) and drops the two context-graph tables from
  20260706193007 whose feature code was never merged and which prod no
  longer has, so fresh replays agree with prod.
- tests/pg/parties-phase0.pg.test.ts pins the extension, a sanity check on
  trigram ranking, and the absence of the graph tables.
- scripts/parties/draw-golden-set.sql is the reproducible, read-only draw
  of the 200-key labelling sample (three strata, md5-ordered) and the
  payee-identity base rate. The drawn rows contain customer voucher text
  and are kept in gitignored dev_docs, never in this public repo.
- scripts/parties/README.md records the label vocabulary and the numbers
  measured on prod on 2026-09-02.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

* fix(archive): drop the two context-graph tables from the archive contract

The migration in this PR removes graph_counterparties and
graph_transaction_counterparties, so the full-archive contract must stop
classifying them: tests/schema/no-phantom-columns.test.ts asserts that
every classified table exists in the migration replay, and the live-DB
twin in tests/pg/full-archive-coverage.pg.test.ts asserts the same against
information_schema.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

---------

Co-authored-by: Jakob Wennberg <311770904+jakobwennberg-oss@users.noreply.github.com>
Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>

scripts/parties/README.md

@@ -0,0 +1,58 @@
+# Parties, phase 0: measure before building
+
+Phase 0 of the Kontakter plan (design doc: the "Vem du gör affärer med"
+artifact, 2026-09). Nothing in this phase changes product behaviour. It makes
+the resolver measurable so the model steps in later phases can be judged
+against labelled data instead of opinions.
+
+## What ships in this phase
+
+- Migration `20260902120000`: enables `pg_trgm` (trigram blocking of
+  counterparty keys) and drops the two context-graph tables whose feature code
+  was never merged and which prod no longer has. Test:
+  `tests/pg/parties-phase0.pg.test.ts`.
+- `draw-golden-set.sql`: the reproducible draw of the labelling sample and the
+  payee-change base rate. Read-only.
+
+## The golden set (never in git)
+
+The draw returns customer voucher text, which can contain person names. The
+repository is public, so the drawn rows and the labels live in
+`dev_docs/parties/golden/` (gitignored). Only aggregates and this vocabulary
+are versioned.
+
+Label vocabulary for the pre-classifier, one per key:
+
+| label | meaning | example |
+|---|---|---|
+| `party` | a real counterpart the company pays or invoices | `Levfakt Beijer Byggmaterial AB (2089)`, `Loopia` |
+| `category` | an expense description with no counterpart in it | `Inköp av varor`, `Banktjänster`, `Fika` |
+| `payroll` | salary, benefits, expense claims to a person | `Löneutbetalning: 2024.M.01 - anställd: 5` |
+| `adjustment` | periodisering, kostnadsföring, omföring, lagerförändring | `Periodisering av leverantörsfaktura 1311` |
+| `authority` | Skatteverket, Bolagsverket, Transportstyrelsen, kommun | `Bolagsverket ändra bolagsordning` |
+| `bank` | bank fees and bank products | `Baspaket Bank`, `Avgift Amex` |
+| `intermediary` | payment rail or marketplace, not the counterpart | `Klarna`, `Stripe`, `Zettle`, `AmazonMktplc` |
+| `unsure` | cannot tell from the text alone | |
+
+A key whose text names a vendor but is booked as a category still gets
+`party`: the resolver decides identity, the account comes from the ledger.
+
+## Measured on prod, 2026-09-02
+
+- 113,032 imported expense vouchers across 385 real companies; 99.5% yield a
+  key; 71% belong to a key that repeats within the company.
+- Payee-identity base rate: 365 suppliers have an established bankgiro on two
+  or more documents; 13 of them (3.6%) carry two established bankgiro numbers,
+  with an average span of 29 days between them, which reads as parallel
+  accounts or OCR variance rather than changes over time. Any distinct value
+  at all: 8.4%. Consequence for the signal: a new bankgiro is rare enough to
+  warrant a "verify" sentence, never a block, and a value must be seen on two
+  documents before it counts as known.
+
+## Still to confirm in this phase
+
+- Which registration flags the free SCB Företagsregistret API exposes
+  (moms, arbetsgivare, F-skatt). This decides how much of rung three can skip
+  TIC.
+- The pre-classifier and selection agreement bars (0.85 on the labelled set)
+  are enforced by the shadow harness in phase 1, not here.