feat: add performance optimization indexes (bd-bha9, bd-a9y3, bd-jke6, bd-8x3w, bd-lk39)

Migration 026_additional_indexes adds 5 indexes identified during schema review (bd-h0we):

- idx_issues_updated_at: For GetStaleIssues date filtering
- idx_issues_status_priority: For common list query patterns
- idx_labels_label_issue: Covering index for label lookups
- idx_dependencies_issue_type: For blocked issues queries
- idx_events_issue_type: For close reason queries

These indexes improve query performance for common operations,
particularly at scale (10K+ issues).

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

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

internal/storage/sqlite/migrations/026_additional_indexes.go

@@ -0,0 +1,58 @@
+package migrations
+
+import (
+	"database/sql"
+	"fmt"
+)
+
+// MigrateAdditionalIndexes adds performance optimization indexes identified
+// during schema review (bd-h0we).
+//
+// Indexes added:
+//   - idx_issues_updated_at: For GetStaleIssues date filtering (bd-bha9)
+//   - idx_issues_status_priority: For common list query patterns (bd-a9y3)
+//   - idx_labels_label_issue: Covering index for label lookups (bd-jke6)
+//   - idx_dependencies_issue_type: For blocked issues queries (bd-8x3w)
+//   - idx_events_issue_type: For close reason queries (bd-lk39)
+func MigrateAdditionalIndexes(db *sql.DB) error {
+	indexes := []struct {
+		name string
+		sql  string
+	}{
+		// Issues table indexes
+		{
+			name: "idx_issues_updated_at",
+			sql:  `CREATE INDEX IF NOT EXISTS idx_issues_updated_at ON issues(updated_at)`,
+		},
+		{
+			name: "idx_issues_status_priority",
+			sql:  `CREATE INDEX IF NOT EXISTS idx_issues_status_priority ON issues(status, priority)`,
+		},
+
+		// Labels table covering index
+		{
+			name: "idx_labels_label_issue",
+			sql:  `CREATE INDEX IF NOT EXISTS idx_labels_label_issue ON labels(label, issue_id)`,
+		},
+
+		// Dependencies table composite index
+		{
+			name: "idx_dependencies_issue_type",
+			sql:  `CREATE INDEX IF NOT EXISTS idx_dependencies_issue_type ON dependencies(issue_id, type)`,
+		},
+
+		// Events table composite index
+		{
+			name: "idx_events_issue_type",
+			sql:  `CREATE INDEX IF NOT EXISTS idx_events_issue_type ON events(issue_id, event_type)`,
+		},
+	}
+
+	for _, idx := range indexes {
+		if _, err := db.Exec(idx.sql); err != nil {
+			return fmt.Errorf("failed to create index %s: %w", idx.name, err)
+		}
+	}
+
+	return nil
+}