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/submission.go

@@ -61,6 +61,62 @@ func (s *Store) GetSubmission(ctx context.Context, id string) (Submission, error
 	return sub, nil
 }
 
+// SubmissionSummary ist eine Submission plus einer Kurzfassung ihrer
+// aktuell gültigen Findings, wie sie eine Übersichtsliste braucht (ohne
+// für jede Zeile extra ListCurrentFindings aufzurufen).
+type SubmissionSummary struct {
+	Submission
+	FindingCount    int
+	HighestSeverity string // "" wenn keine Findings
+}
+
+// ListSubmissionsForAccount liefert alle Beiträge eines Mandanten,
+// neueste zuerst, mit Findings-Kurzfassung.
+func (s *Store) ListSubmissionsForAccount(ctx context.Context, accountID string) ([]SubmissionSummary, error) {
+	rows, err := s.Pool.Query(ctx, `
+		SELECT
+			s.id, s.account_id, s.platform, s.post_type, s.caption, s.status, s.created_at, s.updated_at,
+			COUNT(f.id) AS finding_count,
+			COALESCE(MAX(CASE f.severity WHEN 'hoch' THEN 3 WHEN 'mittel' THEN 2 WHEN 'niedrig' THEN 1 ELSE 0 END), 0) AS severity_rank
+		FROM submission s
+		LEFT JOIN finding f
+			ON f.submission_id = s.id
+			AND NOT EXISTS (SELECT 1 FROM finding f2 WHERE f2.supersedes = f.id)
+		WHERE s.account_id = $1
+		GROUP BY s.id
+		ORDER BY s.created_at DESC
+	`, accountID)
+	if err != nil {
+		return nil, fmt.Errorf("store: list submissions for account: %w", err)
+	}
+	defer rows.Close()
+
+	var out []SubmissionSummary
+	for rows.Next() {
+		var sub SubmissionSummary
+		var severityRank int
+		if err := rows.Scan(
+			&sub.ID, &sub.AccountID, &sub.Platform, &sub.PostType, &sub.Caption, &sub.Status, &sub.CreatedAt, &sub.UpdatedAt,
+			&sub.FindingCount, &severityRank,
+		); err != nil {
+			return nil, fmt.Errorf("store: scan submission summary: %w", err)
+		}
+		switch severityRank {
+		case 3:
+			sub.HighestSeverity = "hoch"
+		case 2:
+			sub.HighestSeverity = "mittel"
+		case 1:
+			sub.HighestSeverity = "niedrig"
+		}
+		out = append(out, sub)
+	}
+	if err := rows.Err(); err != nil {
+		return nil, fmt.Errorf("store: list submissions for account: %w", err)
+	}
+	return out, 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