fix(config): escape LIKE wildcards in registration search filter (parity with ListCloudAccounts)

[cristim]

Surfaced by the full-codebase SQL injection audit (2026-05-26). The audit found zero SQLi vulnerabilities but one suspicious LIKE-wildcard escape gap.

Defect

internal/config/store_postgres_registrations.go:101-107:

if filter.Search != "" {
    conditions = append(conditions, fmt.Sprintf(
        "(account_name ILIKE $%d OR external_id ILIKE $%d OR contact_email ILIKE $%d)",
        idx, idx, idx,
    ))
    args = append(args, "%"+filter.Search+"%")
    idx++
}

filter.Search is bound as $N so this is not SQL injection — the value never enters SQL text. BUT % / _ / \ in filter.Search are not escaped. A user passing %@gmail.com matches every email containing @gmail.com rather than literally containing %@gmail.com. Enumeration / minor information-disclosure risk.

Fix

Mirror the working pattern in internal/config/store_postgres.go:1974 (ListCloudAccounts search filter): escape %, _, \ before wrapping with %...% and add ESCAPE '\\' to the ILIKE.

Add a regression test asserting that filter.Search = "%foo" matches the literal %foo substring, not any foo-containing string.

[cristim]

Verified shipped: PR #720 (commit 5c15dd3, merged 2026-05-27) is in feat/multicloud-web-frontend. store_postgres_registrations.go now escapes %, _, \ in Search filter before ILIKE wrap, matching the pattern in ListCloudAccounts. Regression test added in store_postgres_pgxmock_test.go.