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credit model is fully superseded by PAYG (`wallet_ledger`) — confirmed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`.

frontend/editor/public/locales/en-GB/translation.toml

@@ -2756,7 +2756,6 @@ tooltip = "Pick colour from screen"
 title = "Choose colour"
 
 [common]
-available = "available"
 back = "Back"
 cancel = "Cancel"
 close = "Close"
@@ -2774,7 +2773,6 @@ previous = "Previous"
 refresh = "Refresh"
 retry = "Retry"
 save = "Save"
-used = "used"
 
 [compare]
 clearSelected = "Clear selected"
@@ -3020,9 +3018,7 @@ title = "Upgrade Guest Account"
 upgradeButton = "Upgrade Account"
 
 [config.apiKeys]
-chartAriaLabel = "Credits usage: included {{includedUsed}} of {{includedTotal}}, purchased {{purchasedUsed}} of {{purchasedTotal}}"
 copyKeyAriaLabel = "Copy API key"
-creditsRemaining = "Credits Remaining"
 description = "Your API key for accessing Stirling's suite of PDF tools. Copy it to your project or refresh to generate a new one."
 docsDescription = "Learn more about integrating with Stirling PDF:"
 docsLink = "API Documentation"
@@ -3030,14 +3026,9 @@ docsTitle = "API Documentation"
 generateError = "We couldn't generate your API key."
 goToAccount = "Go to Account"
 guestInfo = "Guest users do not receive API keys. Create an account to get an API key you can use in your applications."
-includedCredits = "Included credits"
 intro = "Use your API key to programmatically access Stirling PDF's processing capabilities."
 label = "API Key"
-lastApiUse = "Last API Use"
-nextReset = "Next Reset"
-overlayMessage = "Generate a key to see credits and available credits"
 publicKeyAriaLabel = "Public API key"
-purchasedCredits = "Purchased credits"
 refreshAriaLabel = "Refresh API key"
 schemaLink = "API Schema Reference"
 usage = "Include this key in the X-API-KEY header with all API requests."