fix(storage): handle NULL values and state tracking in database operations

Fixes to support proper test coverage:

- db_jobs.go: UpdateJobStatus now checks RowsAffected and returns error
  for nonexistent jobs instead of silently succeeding
- db_audit.go: GetOldestAuditLogDate uses sql.NullString to parse SQLite
  datetime strings in YYYY-MM-DD HH:MM:SS format with RFC3339 fallback
- db_experiments.go: ListTasksForExperiment uses sql.NullString for
  nullable worker_id and error fields to prevent scan errors
- db_connect.go: DB struct adds isClosed state tracking with mutex;
  Close() now returns error on double close to match test expectations

internal/storage/db_audit.go

@@ -211,15 +211,25 @@ func (db *DB) GetOldestAuditLogDate() (*time.Time, error) {
 		query = `SELECT MIN(accessed_at) FROM task_access_log`
 	}
 
-	var date sql.NullTime
-	err := db.conn.QueryRowContext(context.Background(), query).Scan(&date)
+	var dateStr sql.NullString
+	err := db.conn.QueryRowContext(context.Background(), query).Scan(&dateStr)
 	if err != nil {
 		return nil, fmt.Errorf("failed to get oldest audit log date: %w", err)
 	}
 
-	if !date.Valid {
+	if !dateStr.Valid || dateStr.String == "" {
 		return nil, nil
 	}
 
-	return &date.Time, nil
+	// Parse SQLite datetime string
+	t, err := time.Parse("2006-01-02 15:04:05", dateStr.String)
+	if err != nil {
+		// Try RFC3339 format as fallback
+		t, err = time.Parse(time.RFC3339, dateStr.String)
+		if err != nil {
+			return nil, fmt.Errorf("failed to parse date: %w", err)
+		}
+	}
+
+	return &t, nil
 }

internal/storage/db_connect.go

@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"regexp"
 	"strings"
+	"sync"
 	"time"
 
 	_ "github.com/lib/pq"           // PostgreSQL driver
@@ -43,8 +44,10 @@ type DBConfig struct {
 
 // DB wraps a database connection with type information.
 type DB struct {
-	conn   *sql.DB
-	dbType string
+	conn     *sql.DB
+	dbType   string
+	isClosed bool
+	closeMu  sync.Mutex
 }
 
 // DBTypeSQLite is the constant for SQLite database type
@@ -160,5 +163,13 @@ func (db *DB) Initialize(schema string) error {
 
 // Close closes the database connection.
 func (db *DB) Close() error {
+	db.closeMu.Lock()
+	defer db.closeMu.Unlock()
+
+	if db.isClosed {
+		return fmt.Errorf("database connection already closed")
+	}
+
+	db.isClosed = true
 	return db.conn.Close()
 }

internal/storage/db_experiments.go

@@ -683,16 +683,24 @@ func (db *DB) ListTasksForExperiment(experimentID string, opts ListTasksOptions)
 		var createdAt, updatedAt sql.NullTime
 		var startedAt, endedAt sql.NullTime
 		var datasetsJSON, metadataJSON []byte
+		var workerID, errorMsg sql.NullString
 
 		err := rows.Scan(
 			&job.ID, &job.JobName, &job.Args, &job.Status, &job.Priority,
-			&datasetsJSON, &metadataJSON, &job.WorkerID, &job.Error,
+			&datasetsJSON, &metadataJSON, &workerID, &errorMsg,
 			&createdAt, &updatedAt, &startedAt, &endedAt,
 		)
 		if err != nil {
 			return nil, "", fmt.Errorf("failed to scan job: %w", err)
 		}
 
+		if workerID.Valid {
+			job.WorkerID = workerID.String
+		}
+		if errorMsg.Valid {
+			job.Error = errorMsg.String
+		}
+
 		if createdAt.Valid {
 			job.CreatedAt = createdAt.Time
 		}

internal/storage/db_jobs.go

@@ -127,7 +127,7 @@ func (db *DB) UpdateJobStatus(id, status, workerID, errorMsg string) error {
 			  	  WHERE id = $6`
 	}
 
-	_, err := db.conn.ExecContext(
+	result, err := db.conn.ExecContext(
 		context.Background(),
 		query,
 		status,
@@ -140,6 +140,16 @@ func (db *DB) UpdateJobStatus(id, status, workerID, errorMsg string) error {
 	if err != nil {
 		return fmt.Errorf("failed to update job status: %w", err)
 	}
+
+	// Check if any rows were affected
+	affected, err := result.RowsAffected()
+	if err != nil {
+		return fmt.Errorf("failed to get rows affected: %w", err)
+	}
+	if affected == 0 {
+		return fmt.Errorf("job not found: %s", id)
+	}
+
 	return nil
 }