feat: add store CRUD methods for submission/extraction/finding/evidence

Create/Get for submission (including the one legitimate status
transition — submission is not append-only, unlike the other three),
Create/GetLatest for extraction and evidence_package, Create for
finding plus ListCurrentFindings which applies the anti-join documented
in the migration (a finding referenced by another row's `supersedes`
is not "current").

Tested against real Postgres, including that the append-only trigger
still rejects UPDATE on evidence_package via this new code path, and
that ListCurrentFindings actually hides a finding once a correction
supersedes it.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

internal/store/extraction.go

@@ -0,0 +1,56 @@
+package store
+
+import (
+	"context"
+	"fmt"
+	"time"
+)
+
+// Extraction ist das Ergebnis der Stufe-1-Extraktion für einen Beitrag.
+// Payload ist das rohe, vom Modell gelieferte JSON — nicht eine
+// abgeleitete Repräsentation. Append-only: siehe Migration.
+type Extraction struct {
+	ID            string
+	SubmissionID  string
+	Payload       []byte
+	ModelVersion  string
+	PromptVersion string
+	CreatedAt     time.Time
+}
+
+// CreateExtraction speichert eine Extraktion. payload ist das rohe
+// JSON, wie es das Modell zurückgegeben hat (siehe extract.Result.RawJSON).
+func (s *Store) CreateExtraction(ctx context.Context, submissionID string, payload []byte, modelVersion, promptVersion string) (Extraction, error) {
+	var e Extraction
+	err := s.Pool.QueryRow(ctx, `
+		INSERT INTO extraction (submission_id, payload, model_version, prompt_version)
+		VALUES ($1, $2, $3, $4)
+		RETURNING id, submission_id, payload, model_version, prompt_version, created_at
+	`, submissionID, payload, modelVersion, promptVersion).Scan(
+		&e.ID, &e.SubmissionID, &e.Payload, &e.ModelVersion, &e.PromptVersion, &e.CreatedAt,
+	)
+	if err != nil {
+		return Extraction{}, fmt.Errorf("store: create extraction: %w", err)
+	}
+	return e, nil
+}
+
+// GetLatestExtraction liest die zuletzt erzeugte Extraktion für einen
+// Beitrag (append-only: es kann mehrere geben, z. B. bei einer erneuten
+// Prüfung — die aktuellste zählt).
+func (s *Store) GetLatestExtraction(ctx context.Context, submissionID string) (Extraction, error) {
+	var e Extraction
+	err := s.Pool.QueryRow(ctx, `
+		SELECT id, submission_id, payload, model_version, prompt_version, created_at
+		FROM extraction
+		WHERE submission_id = $1
+		ORDER BY created_at DESC
+		LIMIT 1
+	`, submissionID).Scan(
+		&e.ID, &e.SubmissionID, &e.Payload, &e.ModelVersion, &e.PromptVersion, &e.CreatedAt,
+	)
+	if err != nil {
+		return Extraction{}, fmt.Errorf("store: get latest extraction: %w", err)
+	}
+	return e, nil
+}