Fix N+1 query pattern in export operations (bd-rcmg)

**Problem**: Export operations called GetLabels() and GetIssueComments()
in a loop for each issue, creating N+1 query pattern. For 100 issues
this created 201 queries instead of 3-5.

**Solution**:
- Added GetCommentsForIssues() batch method to storage interface
- Implemented batch method in SQLite and memory storage backends
- Updated handleExport() and triggerExport() to use batch queries
- Added comprehensive tests for batch operations

**Impact**: Query count reduced from ~201 to ~3-5 for 100 issues.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

internal/storage/storage.go

@@ -51,6 +51,7 @@ type Storage interface {
 	// Comments
 	AddIssueComment(ctx context.Context, issueID, author, text string) (*types.Comment, error)
 	GetIssueComments(ctx context.Context, issueID string) ([]*types.Comment, error)
+	GetCommentsForIssues(ctx context.Context, issueIDs []string) (map[string][]*types.Comment, error)
 
 	// Statistics
 	GetStatistics(ctx context.Context) (*types.Statistics, error)