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/finding.go

@@ -27,8 +27,14 @@ type Finding struct {
 	CreatedAt    time.Time
 }
 
-// CreateFinding speichert ein Finding.
+// CreateFinding speichert ein Finding. sources ist NOT NULL in der DB
+// (TEXT[]) — ein nil-Slice (z. B. eine Regel ohne fundstelle-Eintrag)
+// würde als SQL-NULL ankommen und mit einer wenig hilfreichen Constraint-
+// Fehlermeldung abgelehnt; hier stattdessen auf eine leere Liste normiert.
 func (s *Store) CreateFinding(ctx context.Context, submissionID string, extractionID *string, ruleID string, ruleVersion int, severity, title, fix string, sources []string) (Finding, error) {
+	if sources == nil {
+		sources = []string{}
+	}
 	var f Finding
 	err := s.Pool.QueryRow(ctx, `
 		INSERT INTO finding (submission_id, extraction_id, rule_id, rule_version, severity, title, fix, sources)