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/sqlite_test.go

@@ -0,0 +1,13 @@
+package database
+
+import "testing"
+
+func TestSQLiteStore(t *testing.T) {
+	runStoreTests(t, func(t *testing.T) Store {
+		store, err := NewSQLiteStore(":memory:")
+		if err != nil {
+			t.Fatalf("Failed to create SQLite store: %v", err)
+		}
+		return store
+	})
+}