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/submission.go

@@ -0,0 +1,66 @@
+package store
+
+import (
+	"context"
+	"fmt"
+	"time"
+)
+
+// Submission ist ein eingereichter Beitrag.
+type Submission struct {
+	ID        string
+	Platform  string
+	PostType  string
+	Caption   string
+	Status    string
+	CreatedAt time.Time
+	UpdatedAt time.Time
+}
+
+// CreateSubmission legt einen neuen Beitrag an (Status "draft").
+func (s *Store) CreateSubmission(ctx context.Context, platform, postType, caption string) (Submission, error) {
+	var sub Submission
+	err := s.Pool.QueryRow(ctx, `
+		INSERT INTO submission (platform, post_type, caption)
+		VALUES ($1, $2, $3)
+		RETURNING id, platform, post_type, caption, status, created_at, updated_at
+	`, platform, postType, caption).Scan(
+		&sub.ID, &sub.Platform, &sub.PostType, &sub.Caption, &sub.Status, &sub.CreatedAt, &sub.UpdatedAt,
+	)
+	if err != nil {
+		return Submission{}, fmt.Errorf("store: create submission: %w", err)
+	}
+	return sub, nil
+}
+
+// GetSubmission liest einen Beitrag anhand seiner ID.
+func (s *Store) GetSubmission(ctx context.Context, id string) (Submission, error) {
+	var sub Submission
+	err := s.Pool.QueryRow(ctx, `
+		SELECT id, platform, post_type, caption, status, created_at, updated_at
+		FROM submission WHERE id = $1
+	`, id).Scan(
+		&sub.ID, &sub.Platform, &sub.PostType, &sub.Caption, &sub.Status, &sub.CreatedAt, &sub.UpdatedAt,
+	)
+	if err != nil {
+		return Submission{}, fmt.Errorf("store: get submission: %w", err)
+	}
+	return sub, nil
+}
+
+// SetSubmissionStatus setzt den Status eines Beitrags (submission ist,
+// anders als extraction/finding/evidence_package, NICHT append-only —
+// der Lebenszyklus draft → checked → published → archived ist eine
+// normale Zustandsänderung, kein Beweis-Eintrag).
+func (s *Store) SetSubmissionStatus(ctx context.Context, id, status string) error {
+	tag, err := s.Pool.Exec(ctx, `
+		UPDATE submission SET status = $2, updated_at = now() WHERE id = $1
+	`, id, status)
+	if err != nil {
+		return fmt.Errorf("store: set submission status: %w", err)
+	}
+	if tag.RowsAffected() == 0 {
+		return fmt.Errorf("store: set submission status: submission %s nicht gefunden", id)
+	}
+	return nil
+}