Fix golangci-lint v2 compliance, make all passes clean

- Fix 314 errcheck violations (blank identifier for unrecoverable errors)
- Fix errorlint violation (errors.Is for io.EOF)
- Remove unused serveL7Route test helper
- Simplify Duration.Seconds() selectors in tests
- Remove unnecessary fmt.Sprintf in test
- Migrate exclusion rules from issues.exclusions to linters.exclusions (v2 schema)
- Add gosec test exclusions (G115, G304, G402, G705)
- Disable fieldalignment govet analyzer (optimization, not correctness)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/db/l7policies.go

@@ -19,7 +19,7 @@ func (s *Store) ListL7Policies(routeID int64) ([]L7Policy, error) {
 	if err != nil {
 		return nil, fmt.Errorf("querying l7 policies: %w", err)
 	}
-	defer rows.Close()
+	defer func() { _ = rows.Close() }()
 
 	var policies []L7Policy
 	for rows.Next() {