feat(db): embedded goose migrations fallback (U10a first slice)

- deploy/postgres (package dbassets) embeds the full goose chain;
  RunMigrations prefers the on-disk dir (dev behavior unchanged) and falls
  back to the embedded FS when the source tree is absent (single-binary
  deployments per plan U10/KTD16)
- goose SetBaseFS global state reset after use
- TestRunMigrationsEmbeddedFallback verified against a throwaway isolated
  pgvector container: nonexistent dir -> embedded chain -> version 78,
  migration gate count agrees

internal/db/migrate.go

@@ -9,6 +9,7 @@ import (
 	"sort"
 	"strings"
 
+	"chat-agent-system/deploy/postgres"
 	"github.com/jackc/pgx/v5/pgxpool"
 	"github.com/jackc/pgx/v5/stdlib"
 	"github.com/pressly/goose/v3"
@@ -25,8 +26,9 @@ func RunMigrations(ctx context.Context, pool *pgxpool.Pool, migrationsDir string
 	if err != nil {
 		return err
 	}
+	useDisk := true
 	if _, err := os.Stat(absDir); err != nil {
-		return fmt.Errorf("db migrate: directory %s: %w", absDir, err)
+		useDisk = false
 	}
 
 	db := stdlib.OpenDB(*pool.Config().ConnConfig)
@@ -37,8 +39,19 @@ func RunMigrations(ctx context.Context, pool *pgxpool.Pool, migrationsDir string
 	}
 	goose.SetTableName(schemaTable)
 
-	if err := goose.UpContext(ctx, db, absDir); err != nil {
-		return fmt.Errorf("db migrate up: %w", err)
+	if useDisk {
+		if err := goose.UpContext(ctx, db, absDir); err != nil {
+			return fmt.Errorf("db migrate up: %w", err)
+		}
+		return nil
+	}
+
+	// U10a：源码目录不存在（单文件部署）时回退内嵌迁移链。
+	// goose SetBaseFS 是全局状态；用后复位，避免污染后续磁盘调用。
+	goose.SetBaseFS(dbassets.MigrationsFS)
+	defer goose.SetBaseFS(nil)
+	if err := goose.UpContext(ctx, db, dbassets.MigrationsDir); err != nil {
+		return fmt.Errorf("db migrate up (embedded): %w", err)
 	}
 	return nil
 }

internal/db/migrate_test.go

@@ -278,3 +278,21 @@ func rewriteDatabaseName(dsn, name string) string {
 	}
 	return dsn
 }
+
+func TestRunMigrationsEmbeddedFallback(t *testing.T) {
+	dsn := os.Getenv("TEST_DATABASE_URL")
+	if dsn == "" {
+		t.Skip("TEST_DATABASE_URL not set; embedded fallback needs PG")
+	}
+	ctx := context.Background()
+	pool, err := pgxpool.New(ctx, dsn)
+	if err != nil {
+		t.Fatalf("connect: %v", err)
+	}
+	defer pool.Close()
+
+	// 不存在的磁盘目录必须触发内嵌回退并成功走完迁移链（幂等）。
+	if err := RunMigrations(ctx, pool, filepath.Join(t.TempDir(), "no-such-dir")); err != nil {
+		t.Fatalf("embedded fallback migrations: %v", err)
+	}
+}