feat: add participant CRUD and per-account submission listing

Participant (the Verantwortungsmatrix — who briefed, who approved) is
not append-only like finding/extraction/evidence_package; getting a
role wrong and correcting it isn't rewriting evidence, so full CRUD is
legitimate here: Create/List/Get/Update/Delete. UpdateParticipant sets
approved_at the first time freigegeben flips to true and never moves it
again on subsequent no-op updates — it marks when approval first
happened, not "last touched".

ListSubmissionsForAccount is the query the upcoming archive overview
needs: every submission for a tenant plus a findings count and highest
severity, computed with the same anti-join ListCurrentFindings already
uses for "currently valid" findings.

Also fixed a real bug this surfaced: CreateFinding let a nil Sources
slice reach a NOT NULL TEXT[] column, which Postgres rejects with an
unhelpful constraint error instead of a clear message. It now normalizes
nil to an empty slice before inserting — matters for any future rule
that ships without a fundstelle entry.

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

internal/store/participant_test.go

@@ -0,0 +1,133 @@
+package store_test
+
+import (
+	"context"
+	"errors"
+	"testing"
+
+	"github.com/netcell-it/deklarix/internal/store"
+)
+
+func TestParticipantCRUD(t *testing.T) {
+	s := openTestStore(t)
+	ctx := context.Background()
+	accID := testAccountID(t, s)
+	sub, err := s.CreateSubmission(ctx, accID, "instagram", "reel", "...")
+	if err != nil {
+		t.Fatalf("CreateSubmission: %v", err)
+	}
+
+	p, err := s.CreateParticipant(ctx, sub.ID, "creator", "Max Mustermann", false, false)
+	if err != nil {
+		t.Fatalf("CreateParticipant: %v", err)
+	}
+	if p.ApprovedAt != nil {
+		t.Fatalf("ApprovedAt should be nil when freigegeben=false, got %v", p.ApprovedAt)
+	}
+
+	list, err := s.ListParticipants(ctx, sub.ID)
+	if err != nil {
+		t.Fatalf("ListParticipants: %v", err)
+	}
+	if len(list) != 1 || list[0].ID != p.ID {
+		t.Fatalf("ListParticipants = %+v, want exactly the created participant", list)
+	}
+
+	got, err := s.GetParticipant(ctx, p.ID)
+	if err != nil {
+		t.Fatalf("GetParticipant: %v", err)
+	}
+	if got.SubmissionID != sub.ID {
+		t.Fatalf("GetParticipant.SubmissionID = %q, want %q", got.SubmissionID, sub.ID)
+	}
+
+	updated, err := s.UpdateParticipant(ctx, p.ID, true, true)
+	if err != nil {
+		t.Fatalf("UpdateParticipant: %v", err)
+	}
+	if !updated.Vorgegeben || !updated.Freigegeben {
+		t.Fatalf("UpdateParticipant did not apply new flags: %+v", updated)
+	}
+	if updated.ApprovedAt == nil {
+		t.Fatal("expected ApprovedAt to be set once freigegeben became true")
+	}
+	firstApproval := *updated.ApprovedAt
+
+	// Ein erneutes Update (weiterhin freigegeben) darf approved_at nicht
+	// verschieben — es haelt fest, wann ZUERST freigegeben wurde.
+	updated2, err := s.UpdateParticipant(ctx, p.ID, true, true)
+	if err != nil {
+		t.Fatalf("UpdateParticipant (2): %v", err)
+	}
+	if !updated2.ApprovedAt.Equal(firstApproval) {
+		t.Fatalf("ApprovedAt changed on a no-op update: %v -> %v", firstApproval, *updated2.ApprovedAt)
+	}
+
+	if err := s.DeleteParticipant(ctx, p.ID); err != nil {
+		t.Fatalf("DeleteParticipant: %v", err)
+	}
+	if _, err := s.GetParticipant(ctx, p.ID); !errors.Is(err, store.ErrNotFound) {
+		t.Fatalf("err after delete = %v, want store.ErrNotFound", err)
+	}
+}
+
+func TestUpdateParticipantNotFound(t *testing.T) {
+	s := openTestStore(t)
+	_, err := s.UpdateParticipant(context.Background(), "00000000-0000-0000-0000-000000000000", true, true)
+	if !errors.Is(err, store.ErrNotFound) {
+		t.Fatalf("err = %v, want store.ErrNotFound", err)
+	}
+}
+
+func TestDeleteParticipantNotFound(t *testing.T) {
+	s := openTestStore(t)
+	err := s.DeleteParticipant(context.Background(), "00000000-0000-0000-0000-000000000000")
+	if !errors.Is(err, store.ErrNotFound) {
+		t.Fatalf("err = %v, want store.ErrNotFound", err)
+	}
+}
+
+func TestListSubmissionsForAccount(t *testing.T) {
+	s := openTestStore(t)
+	ctx := context.Background()
+	accID := testAccountID(t, s)
+
+	subNoFindings, err := s.CreateSubmission(ctx, accID, "instagram", "reel", "organic")
+	if err != nil {
+		t.Fatalf("CreateSubmission: %v", err)
+	}
+	subWithFinding, err := s.CreateSubmission(ctx, accID, "tiktok", "video", "unmarked ad")
+	if err != nil {
+		t.Fatalf("CreateSubmission: %v", err)
+	}
+	// sources bewusst nil statt []string{} — CreateFinding muss das
+	// selbst abfangen (siehe Kommentar dort), nicht der Aufrufer.
+	if _, err := s.CreateFinding(ctx, subWithFinding.ID, nil, "WK-001", 1, "hoch", "t", "f", nil); err != nil {
+		t.Fatalf("CreateFinding: %v", err)
+	}
+
+	// Andere Mandanten duerfen nicht auftauchen.
+	otherAccID := testAccountID(t, s)
+	if _, err := s.CreateSubmission(ctx, otherAccID, "instagram", "reel", "anderer Mandant"); err != nil {
+		t.Fatalf("CreateSubmission (other account): %v", err)
+	}
+
+	list, err := s.ListSubmissionsForAccount(ctx, accID)
+	if err != nil {
+		t.Fatalf("ListSubmissionsForAccount: %v", err)
+	}
+	if len(list) != 2 {
+		t.Fatalf("expected 2 submissions for this account, got %d: %+v", len(list), list)
+	}
+
+	byID := map[string]store.SubmissionSummary{}
+	for _, s := range list {
+		byID[s.ID] = s
+	}
+	if byID[subNoFindings.ID].FindingCount != 0 || byID[subNoFindings.ID].HighestSeverity != "" {
+		t.Errorf("subNoFindings summary = %+v, want 0 findings and no severity", byID[subNoFindings.ID])
+	}
+	if byID[subWithFinding.ID].FindingCount != 1 || byID[subWithFinding.ID].HighestSeverity != "hoch" {
+		t.Errorf("subWithFinding summary = %+v, want 1 finding, severity hoch", byID[subWithFinding.ID])
+	}
+}