feat: add Postgres store with append-only schema and migrations

internal/store connects via pgx and runs golang-migrate migrations
embedded in the binary (go:embed), so Deklarix stays a single binary
despite the move to Postgres. Schema covers the five MVP tables
(submission, asset, extraction, finding, evidence_package, participant).

extraction, finding and evidence_package are append-only by design: a
Postgres trigger rejects UPDATE/DELETE outright, since a corrigible
evidence archive isn't an evidence archive. Corrections to a finding are
new rows whose supersedes column points at the row they replace (set at
INSERT time on the new row, since the trigger blocks UPDATE on the old
one) — "currently valid" findings are the ones no other row supersedes.

scripts/test.sh now spins up a disposable Postgres container so the
store's integration tests (including the append-only guarantee) actually
run on every test.sh/release.sh invocation instead of silently skipping
for lack of DATABASE_URL.

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

internal/store/store.go

@@ -0,0 +1,31 @@
+package store
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/jackc/pgx/v5/pgxpool"
+)
+
+// Store hält den Verbindungspool zur Postgres-Datenbank.
+type Store struct {
+	Pool *pgxpool.Pool
+}
+
+// Open baut den Verbindungspool auf und prüft ihn mit einem Ping.
+func Open(ctx context.Context, databaseURL string) (*Store, error) {
+	pool, err := pgxpool.New(ctx, databaseURL)
+	if err != nil {
+		return nil, fmt.Errorf("store: open pool: %w", err)
+	}
+	if err := pool.Ping(ctx); err != nil {
+		pool.Close()
+		return nil, fmt.Errorf("store: ping: %w", err)
+	}
+	return &Store{Pool: pool}, nil
+}
+
+// Close gibt den Verbindungspool frei.
+func (s *Store) Close() {
+	s.Pool.Close()
+}