test: database layer tests

- Add comprehensive test suite for Store interface
- Test schema initialization, revisions, options, search, declarations, files
- SQLite tests use in-memory database for speed
- PostgreSQL tests require TEST_POSTGRES_CONN environment variable

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/database/postgres_test.go

@@ -0,0 +1,21 @@
+package database
+
+import (
+	"os"
+	"testing"
+)
+
+func TestPostgresStore(t *testing.T) {
+	connStr := os.Getenv("TEST_POSTGRES_CONN")
+	if connStr == "" {
+		t.Skip("TEST_POSTGRES_CONN not set, skipping PostgreSQL tests")
+	}
+
+	runStoreTests(t, func(t *testing.T) Store {
+		store, err := NewPostgresStore(connStr)
+		if err != nil {
+			t.Fatalf("Failed to create PostgreSQL store: %v", err)
+		}
+		return store
+	})
+}