refactor(saas): remove the legacy credits engine (FE + Java) (#6687)

Complete legacy-credits teardown ("Group 3"). The per-user/per-team
credit model is fully superseded by PAYG (`wallet_ledger`) — confirmed
no PAYG code references it. Authorized to also remove the `TeamCredit`
pool + its monthly reset.

## Frontend (saas)
- Deleted `saas/hooks/useCredits.ts`, `apiKeys/hooks/useCredits.ts`,
`types/credits.ts`, `apiKeys/UsageSection.tsx`.
- `UseSession.tsx`: removed credit members (`creditBalance`,
`creditSummary`, `hasSufficientCredits`, `updateCredits`,
`refreshCredits`, `fetchCredits`) + the credit types + global
credit-update callback. **Kept** `isPro`/`refreshProStatus` and the
Supabase auth subscription listener.
- `services/apiClient.ts`: removed the dead `x-credits-remaining`
handler + low-credit plumbing (token-refresh / PAYG / 401 logic
untouched).
- Credit refs removed from `ApiKeys.tsx`, `AppConfigModal.tsx`,
`auth/teamSession.ts`.

## Java (:saas)
**Deleted (15):** `UserCredit`(+repo),
`TeamCredit`(+repo)+`TeamCreditService`, `CreditService`,
`CreditHeaderUtils`, `CreditResetScheduler`, `CreditController`,
`CreditInterceptorConfig`, `UnifiedCreditInterceptor`,
`CreditSuccessAdvice`, `CreditErrorAdvice`, `CreditConsumptionResult` (+
the CreditController test).

**Edited — stripped legacy credit side-effects, preserved
auth/role/AI/PAYG logic:**
- `AiCreate`/`AiProxyController`: dropped the
`X-Credits-Remaining`/`X-Credit-Source` response header (its only
consumer, the desktop credit system, was already removed).
- `SaasTeamService`: dropped UserCredit/TeamCredit init on team-create +
seat-update.
- `SupabaseAuthenticationFilter` / `SupabaseSecurityConfig`: dropped
`getOrCreateUserCredits` on signup + the credit field/CORS header.
- `UserRoleService`: dropped `resetCycleAllocationForRoleChange`;
`ROLE_PRO_USER` grant/revoke preserved.
- proprietary `UserRepository`: dropped
`findUsersWithApiKeyButNoCredits()`.
- Tests updated to drop credit mocks/refs.

## Kept / scope
- `isPro` / `is_pro` RPC / `ROLE_PRO_USER` (that's the separate Group-4
/ EE effort) and **all PAYG** are untouched.
- **No DB tables dropped.** `user_credits`/`team_credits` stay until a
later **gated** migration — which this PR unblocks (the JPA entities
that pinned them are gone).

## Verify
`:saas:compileJava` + `:saas:compileTestJava` pass; FE `tsc --noEmit`
(saas) + eslint clean; 0 stray artifacts; no residual source refs to the
deleted classes.

## Follow-up (not in this PR)
`ErrorTrackingService` (+
`UserErrorTracker`/`ProcessingErrorType`/`CreditsProperties`) is now a
dead island — its only callers were the deleted interceptors. Safe to
delete, but it cascades beyond the credit scope, so it's a separate
tidy-up.

Targets `feat/desktop-cloud-saas-reuse`.

app/proprietary/src/main/java/stirling/software/proprietary/security/database/repository/UserRepository.java

@@ -105,15 +105,6 @@ public interface UserRepository extends JpaRepository<User, Long> {
     Stream<Long> findByUsernameIsNullAndCreatedAtBefore(
             @Param("cutoffDate") LocalDateTime cutoffDate);
 
-    /** Users with an API key but no row in {@code user_credits}. */
-    @Query(
-            value =
-                    "SELECT u.* FROM users u "
-                            + "LEFT JOIN user_credits uc ON uc.user_id = u.user_id "
-                            + "WHERE u.api_key IS NOT NULL AND uc.user_id IS NULL",
-            nativeQuery = true)
-    List<User> findUsersWithApiKeyButNoCredits();
-
     /** Single-shot UPDATE that reassigns a user to a different team. */
     @Modifying
     @Query("UPDATE User u SET u.team.id = :teamId WHERE u.id = :userId")